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.     

汽车制动过程中摩擦层的作用和形成机制

总结了汽车制动过程中摩擦层的生成及其对摩擦性能的影响和摩擦层结构的特征：（1）摩擦层具有组分依赖性,摩擦材料中使用的原材料决定摩擦层的组成并最终决定摩擦性能;（2）摩擦热导致摩擦化学反应发生而在摩擦层产生新物质;（3）摩擦层生成是动态的,具有不断生成、破坏、再生成、再破坏的循环过程;（4）摩擦层结构是变化的,与摩擦温度、压力、速度、气氛等条件有关;（5）摩擦层是可控的,通过控制摩擦层结构可以制备高性能（具有稳定摩擦因数、低磨损和低噪声）摩擦材料。     

沟槽形表面织构对摩擦噪声的影响

用电加工方法在制动盘蠕墨铸铁材料表面加工出沟槽形表面织构(沟槽深度为30μm、宽度为150μm、间距为500μm),采用球—平面接触方式,选取直径为10mm的Si3N4球为对磨副,对沟槽形织构表面和光滑表面进行了摩擦噪声对比试验,研究了沟槽形表面织构对界面摩擦振动噪声的影响。试验结果表明(以下结论只针对本试验选定尺寸规格的沟槽形表面织构):法向载荷对织构表面产生摩擦噪声强度的影响较小,而对光滑表面产生摩擦噪声的水平具有重要影响;沟槽形织构表面在低法向载荷下较光滑表面更易产生摩擦噪声,但随着法向载荷从5N增大到10N,光滑表面产生的摩擦噪声强度迅速增大并与织构表面的接近;沟槽形表面织构使摩擦系统更易产生多频率的摩擦振动,较早地产生摩擦噪声且其主频率成分较复杂;沟槽形织构表面比光滑表面具有较高的摩擦因数和耐磨性,沟槽形织构的存在明显地改变了接触界面摩擦磨损行为和摩擦噪声特性,但其对应关系需要进一步深入研究。     

Friction force microscopy investigations of potassium halide surfaces in ultrahigh vacuum: structure,friction and surface modification

Friction force microscopy measurements on the vacuum‐cleaved (001) surfaces of KF, KCl and KBr have been carried out. All surfaces exhibit atomically flat terraces with monatomic steps aligned preferentially along low‐index lattice directions. Stick‐slip lateral forces with the lattice periodicity are observed on all surfaces. Tip‐sample contact creates higher friction domains on the terraces of all three materials. The structure, topography and degree of friction force contrast of these domains is material dependent. The dependence of friction upon load generally does not coincide with the behavior expected for an elastic contact. We propose that the observed domains result from surface structural changes created by low load tip‐sample contact on these relatively soft materials. This revised version was published online in June 2006 with corrections to the Cover Date.     

Friction behavior of quenched and tempered steel in partial and gross slip conditions in fretting point contact

Tangential traction caused by friction in contacting surfaces is a major factor in fretting fatigue that increases stress levels and leads to a reduction in fatigue life. Friction in fretting contact was studied in partial, mixed and gross slip conditions on quenched and tempered steel. Measurements were made with sphere-on-plane contact geometry for polished and ground surfaces. Friction was evaluated from on-line energy ratio and, after the tests, from wear marks. A maximum friction coefficient of over 1.0 was measured at mixed slip zone with polished surfaces, whereas ground surfaces promote lower values in similar operating conditions. The friction coefficient dependence on load cycles and loading frequency is also presented and briefly discussed. The friction data and understanding thus gained is to be used for evaluation of crack initiation with the numerical fretting fatigue model.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

A predictive analytical friction model from basic theories of interfaces, contacts and dislocations

Friction between crystalline bodies is described in a model that unifies elements of dislocation drag, contact mechanics, and interface theory. An analytic expression for the friction force between solids suggests that dislocation drag accounts for many of the observed phenomena related to solid–solid sliding. Included in this approach are strong arguments for agreement with friction dependence on temperature, velocity, orientation, and more general materials selection effects. It is shown that calculations of friction coefficients for sliding contacts are in good agreement with available experimental values reported from ultrahigh vacuum experiments. Extensions of this model include solutions for common types of dislocation barriers or defects. The effects of third-body solid lubricants, superplasticity, superconductivity, the Aubry transition, and supersonic dislocation motion are all discussed in the framework of dislocation-mediated friction.     

Experimental Research on Friction of Vehicle Tire Rubber

A newly developed tire rubber friction test machine is introduced. Friction test method of tire rubber is provided. Test data of tire rubber friction on concrete and icy road surfaces are obtained and analyzed. The effect of different road surface, ambient temperature, contact pressure, and slip velocity on friction coefficient is apprehended. The dynamic friction is introduced to tire semi-empirical modeling, and the accuracy of the model is improved. A way of forecasting tire property on high-rolling speed using data from low-rolling speed tire test is illustrated.     

材料干滑动摩擦磨损性能的研究进展

介绍高温、高速、载流、气氛、磁场等苛刻条件下材料的干滑动摩擦磨损特性的研究方法和研究结果。得到以下几点结论:材料的干摩擦磨损特性与速度、载荷之间存在着对应关系,当pv值超过临界值后,材料的摩擦因数和磨损率发生突变;摩擦面温度的升高会降低材料的干摩擦磨损性能;环境气氛的改变不影响金属摩擦副pv特性的基本规律,但显著影响其摩擦因数和磨损率数值的大小;电流的存在恶化材料的干摩擦性能;施加磁场可显著改善材料的摩擦磨损性能,并且磁场强度是影响材料干摩擦特性的主要因素。     

A vibration analysis of a turbine blade system damped by dry friction forces

A turbine blade system with dry friction between the shroud ring elements is analysed and results obtained by a finite element method, are presented. Friction forces in the interacting surfaces are described in terms of anisotropic dry friction theory. In the examples, frequencies and mode shapes of undamped free vibrations of the system are analysed. Vibration excitation due to friction and vibration coupling caused by friction anisotropy are described when vibration is initiated by prescribed displacements or velocities.     

Friction characteristics of nanoscale sliding contacts between multi-asperity tips and textured surfaces

Nanoscale sliding contacts of smooth surfaces or between a single asperity and a smooth surface have been widely investigated by molecular dynamics simulations, while there are few studies on the sliding contacts between two rough surfaces. Actually, the friction of two rough surfaces considering interactions between more asperities should be more realistic. By using multiscale method, friction characteristics of two dimensional nanoscale sliding contacts between rigid multi-asperity tips and elastic textured surfaces are investigated. Four nanoscale textured surfaces with different texture shapes are designed, and six multi-asperity tips composed of cylindrical asperities with different radii are used to slide on the textured surfaces. Friction forces are compared for different tips, and effects of the asperity radii on the friction characteristics are investigated. Average friction forces for all the cases are listed and compared, and effects of texture shapes of the textured surfaces are discussed. The results show that textured surface II has a better structure to reduce friction forces. The multi-asperity tips composed of asperities with R=20r0 （r0=0.227 7 nm） or R=30r0 get higher friction forces compared with other cases, and more atoms of the textured surfaces are taken away by these two tips, which are harmful to reduce friction or wear. For the case of R=10ro, friction forces are also high due to large contact areas, but the sliding processes are stable and few atoms are taken away by the tip. The proposed research considers interactions between more asperities to make the model approach to the real sliding contact problems. The results will help to vary or even control friction characteristics by textured surfaces, or provide references to the design of textured surfaces.     

惯性制动条件对铜-石墨材料摩擦性能及第三体的影响

采用粉末冶金技术制备铜-10%石墨烧结材料,通过GF150D型摩擦试验机,在干摩擦状态及制动压力为0.51 MPa的条件下,研究不同制动方式对材料摩擦磨损性能的影响。结果表明,采用从高速到低速分段制动方式(摩擦方式A)时,随着制动速度降低,摩擦表面形成的致密第三体破碎、剥落,机械啮合力增加,摩擦因数提高;同时,摩擦表面温度下降,基体强度提高,磨损率降低。采用从高速到低速连续制动方式(摩擦方式B)的摩擦因数和磨损量均大于摩擦方式A。     

Friction and dry sliding wear behavior of carbon and glass fabric reinforced vinyl ester composites

Friction and dry sliding wear behavior of glass and carbon fabric reinforced vinyl ester composites have been presented. The results show that the coefficient of friction and wear rate increased with increase in load/sliding velocity and depends on type of fabric reinforcement and temperature at the interphase. The excellent tribological characteristics were obtained with carbon fiber in vinyl ester. It is believed that a thin film formed on counterface was seems to be effective in improving the tribological characteristics. The worn surfaces examined through SEM, showed higher levels of broken glass fiber in vinyl ester compared to carbon-vinyl ester composites.     

Actual contact area growth caused by interaction of junctions in dry friction

Experimental data characterizing dry friction processes between strain-hardenable metallic bodies under various normal loads are presented. It is shown that changes both in tangential force and friction coefficient observed in experiments are accompanied by a corresponding growth in the actual contact area (i.e. total area of metallic junctions) between contacting surfaces. Variations of actual contact area are explained as a result of interaction of metallic junctions, caused by their coalescence, which depends on the distances between actual contact area spots. The results obtained reveal some peculiarities of the process seizure.     

水润滑下偶件表面粗糙度对PTFE复合材料摩擦学性能的影响

通过水润滑下的摩擦磨损实验,研究了偶件表面粗糙度对MoS2/PTFE复合材料摩擦学性能的影响,分析了在不同的偶件表面粗糙度下的摩擦学行为.实验结果表明:在水润滑下,一般存在着一个较佳的偶件表面粗糙度范围,在这个范围内可以取得较低的摩擦因数和磨损率;当偶件表面粗糙度高于这个范围时,摩擦磨损机制主要是机械作用;而当偶件表面粗糙度低于这个范围时,则主要是由于分子作用导致摩擦磨损.即当偶件表面粗糙度超出某一范围时,摩擦磨损行为将发生转变.     

镍基Si3N4复合镀层的摩擦磨损特性

通过复合电镀法制备了N i-Si3N4和N i-P-Si3N4两种复合镀层,分别与热处理45#钢组成摩擦副,进行环-环摩擦磨损试验,测试了摩擦因数和磨损量,并观察了磨损表面形貌,探讨了摩擦副的摩擦磨损机理。结果表明,N i-P-Si3N4/45#钢摩擦副的摩擦因数较小,磨损量低,具有良好的减摩耐磨性能。镀层基体的性能明显影响复合镀层的摩擦磨损性能。磨料磨损是N i-Si3N4/45#钢摩擦副的主要磨损形式,导致45#钢的磨损量增大;N i-P-Si3N4镀层对Si3N4颗粒具有良好的把持力,避免了磨料磨损的产生,摩擦副处于稳定的边界润滑摩擦状态。     

Tribological Properties of Alkoxyl Monolayers on Oxide Terminated Silicon

The tribological properties of alkoxyl monolayers on oxide terminated silicon surfaces have been investigated using interfacial force microscopy. For a C18 alkoxyl monolayer, both adhesion and frictional properties are similar to those of a self-assembled monolayer of octadecanethiol on gold. Friction is shown to increase as the alkyl chain length of the molecules decreases. Analysis using contact mechanics models has been carried out to estimate reduced modulus, adhesion energy, and friction shear strength. These interfacial mechanical properties are correlated to molecular structures at the interface.     

硅酸盐粉体作为润滑油添加剂在金属磨损表面成膜机制

在润滑油中添加蛇纹石硅酸盐粉体,采用MM-200摩擦磨损试验机研究了45#钢-45#钢摩擦副磨损表面的自修复陶瓷膜层形成机制,借助SEM及EDAX测试分析自修复陶瓷膜层的表面形貌及表面成分组成。结果表明摩擦能量对硅酸盐添加剂在磨损表面形成自修复膜层有很大的影响:自修复膜层为氧化物陶瓷材料,主要成分来自于硅酸盐添加剂。在低载荷300 N时,摩擦因数减小,硅酸盐添加剂不能转移到磨损表面,不能形成自修复膜层,仅仅起到减磨作用。下试样的失重随磨损时间增加而增加;在试验时间为20 h时,试样失重达到最大值,随后试样的失重反而减小。在载荷为600 N、900 N,试验时间30 h摩擦磨损后,在金属表面形成自修复保护膜,磨损表面比较平整光滑,无明显的片层剥落和犁沟,摩擦发生在自修复陶瓷材料之间,摩擦因数增加。硅酸盐添加剂在机械剪切作用下变形,在金属的磨损表面上铺展,并且在摩擦磨损过程中不断向摩擦表面转移,形成了均匀光滑的自修复膜层。自修复膜层隔离了金属摩擦表面的直接接触,摩擦磨损发生在自修复膜层之间,有效地降低了金属的磨损。     

Friction and a Tribochemical Reaction between Ice and Hexagonal Boron Nitride: A Theoretical Study

Friction between ice-Ih and hexagonal boron nitride (h-BN) has been determined by periodic ab initio calculations. Surfaces of ice-Ih and h-BN were brought into sliding contact, and the interaction energies were calculated as a function of interplanar distance and lateral displacement of the surfaces. The friction between the surfaces was calculated from the interaction energies, producing a friction coefficient of 0.140. Friction is further influenced at high loads by a tribochemical reaction between ice-Ih and h-BN.     

摩擦磨损自修复原理及纳米自修复添加剂研究进展

摩擦磨损导致能源浪费和零部件失效,纳米自修复添加剂能够降低摩擦磨损,延长机械的使用寿命,还可在不拆卸的情况下对机械零件表面进行在线修复,实现终身免大修,给传统的维修带来了全新的理念。探讨了摩擦磨损自修复的原理,指出摩擦磨损自修复的原理主要有摩擦自修复、原位摩擦自修复和摩擦自适应修复等。概述了几种纳米自修复添加剂的研究应用现状,包括纳米金属粉末、纳米金属氧化物、纳米硫化物、纳米硼系和稀土类化合物及其它新型纳米材料。