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.     

Effect of rubber component on the performance of brake friction materials

Various composite friction materials containing 40 vol.% organic binder (phenolic resin plus styrene–butadiene–rubber (SBR)) with varying phenolic-resin/SBR ratio were prepared. The content of phenolic resin in each composite was indicated by the resin value (RV) index ranging between 0 and 100%. The composites with RVs greater than 50% form resin-based friction materials in which the primary binder is the phenolic resin. For RVs less than 50%, the composites become the rubber-based materials where the primary binder is the SBR. The analysis of mechanical properties exhibited that the conformability of the composites increases upon incorporation of SBR. The frictional analysis revealed that type of polymeric binder, i.e. resin or rubber, dominates greatly the frictional behavior of the composites. The increment of friction force and higher improvement in the frictional fade and recovery with sliding velocities are the general features of rubber-based friction materials. It was attributed to the inherent properties of rubber on the viscoelastic response at higher sliding velocities and entropic contribution on the mechanical properties at higher temperatures. The wear rate of resin-based materials and its drum temperature is lower than those of rubber-based materials. It was attributed to the strongly adhered multilayer secondary plateaus formed on the surface of resin-based materials.     

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.     

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.     

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.     

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.     

On friction layer formation in polymer matrix composite materials for brake applications

Due to the complexity of friction phenomena in polymer matrix composites, the friction mechanisms have not been fully understood. This paper concentrates on the characterization of friction layer formation and correlation of friction layer properties to the performance of a recently developed family of polymer matrix composites. It was demonstrated that character of the friction layer determines the friction performance of the investigated composite material. Structure and chemical composition of the friction layer generated on the friction surface significantly differs from the bulk. Mechano-chemical interaction occurring in the friction process is compared to a “non-friction” situation where an “equivalent” apparent temperature and compressive loading, respectively were applied to the same material. No simple relationship exists between composition of the friction layer and bulk material formulation. Phase stability and kinetics of interactions for “friction” and “equivalent non-friction” loading conditions significantly differ.     

Effect of two different rubbers as secondary binders on the friction and wear characteristics of non-asbestos organic (NAO) brake friction materials

Binder provides structural integrity by holding all ingredients in the composition of a brake friction material. The modified binders have played a major role in improving the frictional performance and thermal resistance of the friction material. The present research work evaluates the influence of secondary binders (Nitrile Butadiene rubber (NBR) and Styrene Butadiene rubber (SBR)) on the tribological performance of the friction material using a full-scale inertia brake dynamometer as per JASO C406 standard. Three brake pads were developed by varying the type and composition of secondary rubber binder (5%NBR, 5%SBR and 2.5%NBR + 2.5%SBR) with rest of the ingredients kept unaltered. It was found that the quantity of SBR rubber powder present as secondary binder improved dry and wet recovery. Friction coefficient (μ) exhibited better stability during the fade with the inclusion of both the rubber powders. The friction material with the inclusion of both the NBR and SBR rubber powders exhibited overall better performance than compared to the inclusion of only one secondary binder rubber in the composition. The worn-out surface of the developed friction materials and the counter discs were characterised using FESEM.     

Development of a high-quality rare earth oxide modified resin-based brake material

A high-quality brake material was developed by using different rare earth oxide modified resin-based brake materials and designing the composite material through an orthogonal optimization test. The effect of rare earth oxide on the properties of the resin-based brake material, such as the friction and wear, thermal decay resistance, and recovery, was explored using the constant-speed friction test, Chase friction test and worn surface morphological observation. The results show that rare earth lanthanum oxide can be added to improve the friction and wear properties of the brake material compared to cerium oxide and yttrium oxide. At 100–350°C, the friction coefficient of the developed brake material fluctuates in the range of 0.40–0.50, and the material has high friction coefficient, good thermal decay resistance and favourable wear resistance. The friction coefficient of the optimal formulation can reach the GF level, so its tribological properties are very good and better than the brake pads of automobiles at home and abroad.     

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.     

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.     

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。     

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.