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.     

Studies on friction and transfer layer using inclined scratch

Friction influences the nature of transfer layer formed at the interface between die and sheet during forming. In the present investigation, basic studies were conducted using ‘Inclined Scratch Test’ to understand the mechanism of transfer layer formation during sliding of pins made of an Al–Mg alloy on EN8 steel flats of different surface roughness under dry and lubricated conditions. The surfaces produced can be categorized into three different types: (a) uni-directional (b) 8-ground and (c) random. Rubbing the EN8 flat in a uni-directional manner and a criss-cross manner on emery sheets produced the uni-directional and 8 ground surfaces. The random surfaces were produced by polishing the EN8 flats using various abrasive powders. The influence of the ‘nature of surface roughness’ on material transfer and coefficient of friction were investigated. Scanning Electron Microscopy studies were performed on the contact surfaces of the Al–Mg alloy pins and EN8 steel flats to reveal the morphology of the transfer layer obtained. It was seen that the transfer layer is dependant on the coefficient of friction. The coefficient of friction, which has two components—the adhesion component and the plowing component, is controlled by the ‘nature of surface’. A surface that promotes plane strain conditions near the surfaces increases the plowing component of friction.     

Studies on friction and transfer layer: role of surface texture

Friction influences the nature of transfer layer formed at the interface between tool and metal during sliding. In the present investigation, experiments were conducted using “Inclined Scratch Tester” to understand the effect of surface texture of hard surfaces on coefficient of friction and transfer layer formation. EN8 steel flats were ground to attain surfaces of different textures with different roughness. Then super purity aluminium pins were scratched against the prepared steel flats. Scanning electron micrographs of the contact surfaces of pins and flats were used to reveal the morphology of transfer layer. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the texture of hard surfaces, but independent of surface roughness of hard surfaces. It was observed that on surfaces that promote plane strain conditions near the surface, the transfer of material takes place due to the plowing action of the asperities. But, on a surface that promotes plane stress conditions the transfer layer was more due to the adhesion component of friction. It was observed that the adhesion component increases for surfaces that have random texture but was constant for the other surfaces.     

Influence of tilt angle of plate on friction and transfer layer—A study of aluminium pin sliding against steel plate

In the present investigation, unidirectional grinding marks were attained on the steel plates. Then aluminium (Al) pins were slid at 0.2°, 0.6°, 1.0°, 1.4°, 1.8°, 2.2° and 2.6° tilt angles of the plate with the grinding marks perpendicular and parallel to the sliding direction under both dry and lubricated conditions using a pin-on-plate inclined sliding tester to understand the influence of tilt angle and grinding marks direction of the plate on coefficient of friction and transfer layer formation. It was observed that the transfer layer formation and the coefficient of friction depend primarily on the grinding marks direction of the harder mating surface. Stick-slip phenomenon was observed only under lubricated conditions. For the case of pins slid perpendicular to the unidirectional grinding marks stick-slip phenomenon was observed for tilt angles exceeding 0.6°, the amplitude of which increases with increasing tilt angles. However, for the case of the pins slid parallel to the unidirectional grinding marks the stick-slip phenomena was observed for angles exceeding 2.2°, the amplitude of which also increases with increasing tilt angle. The presence of stick-slip phenomena under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R_a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick–slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick–slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

Wear residues of aramid fibre formation in friction materials

Abstract

Friction and wear characteristics of aramid fibres in the automotive friction material are investigated using a pad on disc type friction tester. By means of SEM and X-ray analysis, the melting aramid fibre and its residues are detected and observed on the rubbing surfaces and lateral surfaces. The aramid fibres suffer the chemical and physical changes due to severe heating induced during braking. When the temperature of the rubbing surfaces of aramid fibres reaches or exceeds the melting point, the fibres on the surface move in the melting state. The melting particles of aramid fibres act as an interface lubricant. So it does not only lead to good wear resistance of the frictional materials, but also causes the coefficient of friction to reduce. The friction and wear mechanism of aramid fibre is discussed and analysed.     

前角和刃倾角的两旋转自由度刃磨方法研究

通过建模和数学推导的方法,确立了一种采用两旋转自由度实现空间任意刀具角度刃磨的数学计算方法,该方法的应用可以解决两旋转自由度机床难以刃磨前角和刃倾角的问题.通过MATLAB编程求解和刃磨试验,验证了模型和推导方程的正确性.     

负前角磨粒磨削过程磨屑形成的有限元仿真分析

磨削过程是由成千上万个磨粒的微小切刃完成的,实验观察和分析磨削过程十分困难.运用有限元法对负前角磨粒的磨削过程进行仿真,分析负前角磨粒磨削时的成屑机理,并将各仿真结果进行比较分析.结果显示,磨粒以小负前角和大负前角磨削时的成屑机理并不相同,与小负前角相比,大负前角磨削时,剪应变、最大等效塑性应变和模型应变能都较大;与前角为正和小负前角时最高温度出现在前刀面上距刀尖一定距离处不同,大负前角时最高温度出现在刀尖和工件接触处.所得结果可为砂轮修整、磨削表面质量研究提供理论依据.     

The effect of the transfer film on the friction and wear of dry bearing materials for a power plant application

Friction and wear tests have been carried out for a number of commercial dry bearing materials containing polytetrafluoroethylene (PTFE), MoS₂ or graphite for a specific turbogenerator application. The effect of operating temperatures up to 300 °C has been investigated. The performance of the materials was strongly related to the formation and stability of a transferred film. PTFE-containing materials offered the most favourable performance over a wide temperature range; the wear rate obeyed a modified form of Archard's adhesive wear law. A simple model for the running-in behaviour of these materials is proposed.     

Undercutting and interference for thread form grinding with a tilt angle

In the form grinding that is widely used to produce precision threads, screws, and gears, the grinding wheel profile is usually calculated from a given tooth profile and the so-called equation of meshing. However, in the presence of undercutting or secondary enveloping, the grinding wheel calculated cannot be guaranteed to produce the desired thread profile. Therefore, this paper proposes a geometric approach to determining the grinding wheel profile and the conditions to avoid undercutting by means of the tilt grinding wheel axis. Specifically, we calculate the thread’s profile based on its relationship to each transverse plane of the grinding wheel in the form grinding process. The grinding wheel profile on each transverse plane is then determined using the shortest distance from all the thread profile points to the grinding wheel revolution axis. Obtaining these distances allows derivation of a formula to calculate the minimum tilt angle that avoids undercutting. This proposed geometric approach is numerically more stable than the conventional equations of meshing, and the derivation of the undercutting equation is straightforward and easy to understand.     

Fundamental studies on the brake friction of resin-based friction materials

The effects of various factors on brake friction were studied on the braking of a cast iron disk by small brake specimens made of resin, resin-asbestos composites and three-component composites. When the disk temperature rises, the resin-based composites containing asbestos and high melting point additive exhibit considerably lower friction during braking following high friction in the initial braking stage. The initial high friction is attributed to the deformation resistance of the resin in the composite. Low friction is attributed to the lubricating action of resin decomposition products in the neighbourhood of the filler raised to a high temperature by frictional heating. Increase of friction toward the end of braking is mainly due to increase of the true contact area as the frictional speed decreases. Variation of the average frictional coefficient during braking with load and the initial speed is generally small. There is no correlation between the mechanical properties and the brake frictional behaviour of resin-based composites.     

磨齿机分度误差传递规律

为预测被加工齿轮的齿距加工精度,研究了Y7125型大平面砂轮磨齿机系统分度误差的传递规律。采用全闭环测量法对用作角度测量基准的正36面棱体进行了高精度标定;基于该正36面棱体和相对测量法在机提取机床36个等分点系统分度误差曲线;最后在磨齿机上进行精密磨齿实验,通过比较齿轮试件的齿距累积偏差与机床原始系统分度误差的差异,研究机床分度误差的传递规律,并通过实验得到磨齿机分度误差传递过程中的不确定度。实验结果表明:采用全闭环测量法标定正36面棱体的测量不确定度达到±0.05;磨齿机系统分度误差传递到被加工齿轮后,齿距累积总偏差由2.1m增大到2.6m,相对误差增加了24%;通过磨齿实验得到磨齿机分度误差传递过程中的不确定度为±0.6m。得到的机床分度误差传递规律可用于预测齿轮的齿距累积加工精度,为制定科学的磨齿工艺提供技术支持。     

Studies on the friction and transfer of semicrystalline polymers

The friction and transfer of various semi-crystalline polymers were studied in several experiments. The cylindrical surfaces of polymers were slid over glass plates at low speed and under constant load. The kinetic friction of PTFE in repeated traverses did not vary with the number of traverses and the transfer of PTFE occurred successively on previously transferred PTFE films. The film transferred at each traverse was extremely thin (< 50 Å). The friction of PTFE decreased with increased humidity in the environment and appeared to be independent of crystalline transitions. Other polymers exhibited higher friction than PTFE and their transfer was generally as small lumps or short streaks. HDPE displayed a very low friction, although the friction of ultrahigh molecular weight polyethylene (UHDPE) was somewhat higher than that of HDPE. With HDPE and UHDPE, as well as with PTFE, long films stretched from one side of the abrasion grooves produced on the polymer frictional surfaces to the other side, like a bridge. The static friction of the three different polymers was very sensitive to the direction of prerubbing on the frictional surfaces and the static friction in sliding parallel to the pre-rubbing direction was much smaller than that perpendicular to it. The roles of the molecular profile and of the banded or spherulitic structure of the polymers in the polymer transfer mechanism are discussed on the basis of the experimental results obtained.     

Analytical determination of friction resistance as a function of normal load and geometry of surface irregularities

A mathematical modeling and simulation of friction during steady state sliding of metals, based on the upper-bound approach, is demonstrated. The existence of wedge-shaped protrusions on the tool surface is assumed. Pressing these protrusions onto the workpiece and sliding the tool along the workpiece produces asperities on the surface of the workpiece. These asperities move in a wave-like motion along the surface layer and cause plastic deformation through a specified depth under the surface. This plastic deformation combines with local friction between the tool and the workpiece along the asperity interface to produce resistance to sliding. The relation between the normal pressure and the sliding resistance is established for the entire range of pressure levels from zero to infinity. The apparent Coulomb coefficient of friction for lower levels of normal pressure and the constant friction factor for excessive load levels are determined. The transition region from Coulomb coefficient of friction to constant friction factor also becomes clear. A mathematical determination is obtained by means of a force equilibrium considering the concept of a contact surface friction ratio. The force of resistance to sliding is related both to the geometry of the asperity of the surface of the tool and to the constant friction factor, which is used for measuring a local frictional force along the interface of each asperity.     

Effect of carbon fiber content on the friction and wear performance of paper-based friction materials

In order to get homogeneous pores distribution of friction materials, four kinds of micron-level carbon fibers reinforced paper-based friction materials were prepared. Experimental results showed that the porosity of samples decreased with the increase of carbon fiber content. Pores formed in micrometer-level fibers reinforced friction materials were more regular than friction materials reinforced by millimeter-level fibers. The tensile strength of samples decreased with the increase of carbon fiber content. The wear rate of samples increased with the increase of carbon fiber content. The sample with 55% carbon fibers exhibited the best friction stability and anti-shudder performance under oil lubricated conditions.     

Study on strengthened layer of workpiece in prestress dry grinding

The prestress dry grinding which combines with prestress grinding and surface strengthening is an innovative approach to substitute conventional grinding process. The residual stress and the thickness of strengthened layer have an important effect on surface performance of workpiece. In order to analyze the effect mechanisms of prestress on residual stress and thickness of strengthened layer, experiment and simulation are carried on for 45 steel. The result of prestress dry grinding experiment shows that the grains in the strengthened layer are refined with increase of prestress. The thickness of strengthened layer tends to decrease first and then increase with increase of prestress. The simulation model of temperature field is created. The effect of temperature field on the thickness of strengthened layer and different grinding parameters especially the prestress on temperature field are analyzed. Then, the relationship between prestress and residual stress on different directions are analyzed. The variation curve of residual stress shows that tensile residual stress can transform to compressive residual stress on the surface of workpiece by controlling the prestress. Besides, compressive residual stress increase first and then decrease with the increase of depth.     

基于CATIA的砂轮R角修正器的设计与分析

砂轮R角修正器是曲轴磨削工艺中,对砂轮R角修正的重要设备;其修正工艺过程直接影响到整个曲轴的加工质量和效率。研究针对原有修正器的缺陷,重新设计了一款新型的砂轮R角修正器。首先,详细地介绍了新修正器的概念设计过程,并通过CATIA电子样机模块对其进行了机构运动仿真和空间干涉检验,找出了初始设计的干涉部位。然后通过优化和仿真检验,重新设计出了砂轮R角修正器的优化模型。最后,将此设计图形进行机加工做出修正器的样品,在实际工作检验中,完全符合加工精度要求,并且大大地提高了工作效率。     

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 model for precision contour grinding of brittle materials

A graphical computer model of the chip geometry resulting from a three-dimensional grinding operation was developed for use in relating the critical depth data obtained from the one-dimensional plunge-grinding technique. This model predicts the resulting surface finish and calculates the theoretical roughness and the final chip geometry for a precision grinding operation. The model is based on euclidean geometry at the intersection of the surfaces of two solid objects. This model was programmed to calculate the remaining surface height as the wheel progresses across the part. The output of the surface profile for successive cuts can be subtracted to illustrate the shape of the chip removed for each revolution of the grinding wheel. Chip geometry as influenced by depth of cut, feed rate, and tool shape was shown to be an important parameter in diamond turning of brittle materials. Similar relationships are developed for the additional geometric complexities of a precision grinding operation. The theoretical surface features are then compared with the actual features generated by grinding brittle materials.