Comprehensive model for scale effects in friction due to adhesion and two- and three-body deformation (plowing)

The coefficient of friction is a sum of the adhesional component and two- and three-body deformation (plowing) components. Scale effects in dry friction at macro- to nanoscale are considered. All components depend on the real area of contact (dependent upon surface roughness and mechanical properties) and shear strength during sliding. In the adhesional component, elastic and plastic deformation and single- and multiple-asperity contacts are considered. For multiple-asperity predominantly plastic contacts, the real area of contact is scale dependent, due to the effect of strain-gradient plasticity, and decreases with decreasing scale, whereas the shear strength is scale-dependent due to the effect of the dislocation-assisted sliding, and increases with decreasing scale. The two-body deformation component increases with decreasing scale due to increasing average asperity slope, an important parameter of interest. The three-body deformation component decreases with decreasing scale, due to decreasing probability for a particle to be trapped at the interface, although the shear strength increases. Transition from elastic deformation to plastic deformation (plowing) is considered. Comparisons of the model with experimental data are presented.     

单晶Cu表面黏-滑效应的分子动力学模拟

运用分子动力学方法模拟了Cu单晶在微摩擦过程中的黏-滑效应．模拟结果表明：在原子尺度,摩擦表面的原子排列较规则,摩擦力曲线为大小锯齿的周期变化．这种黏-滑效应可以解释为位错机制,即摩擦表面间位错的产生与消失的过程．大小锯齿的峰值受载荷、滑动速度、接触面两侧的晶格常数及晶格位向差等多个因素的影响．载荷越大,针尖滑动时移动的原子数量越多,接触面两侧的原子排列越不规则,则小锯齿的峰值越小,并随着载荷的增大而逐渐消失．摩擦力曲线中的小锯齿峰值与滑动速度呈线性关系．不同材料的接触面和不同滑动方向的黏-滑现象并不相同．摩擦力曲线的变化周期取决于滑动过程中基体沿滑动方向的晶格常数．     

金属晶体间微观滑动摩擦特性的分子动力学模拟

根据独立振子模型的能量耗散机理,以光滑干摩擦接触平面为对象,利用金属晶体的强体积效应特征,构造了简化计算的界面势能模型,分析了在界面摩擦状态下能量非连续耗散过程,建立了简化计算滑动摩擦力与摩擦因数模型。从瞬间原子位置图和系统能量方面,进行计算模拟和试验对比验证,结果表明：当接触界面势能处在某个谷底时,滑块受静摩擦力,界面间发生相对运动,滑块变为受滑动摩擦力作用;摩擦力按基本不变的斜率增加到某一值后突然降低,而后变化呈现一定的周期性;随着滑动的逐渐进行,运动体与基体表面的温度逐渐增加而后趋于稳定,两表面存在温度差;滑动后界面平均接触压力明显下降,而后趋于平缓。     

金属晶体间微观滑动摩擦特性的分子动力学模拟

根据独立振子模型的能量耗散机理,以光滑干摩擦接触平面为对象,利用金属晶体的强体积效应特征,构造了简化计算的界面势能模型,分析了在界面摩擦状态下能量非连续耗散过程,建立了简化计算滑动摩擦力与摩擦因数模型。从瞬间原子位置图和系统能量方面,进行计算模拟和试验对比验证,结果表明:当接触界面势能处在某个谷底时,滑块受静摩擦力,界面间发生相对运动,滑块变为受滑动摩擦力作用;摩擦力按基本不变的斜率增加到某一值后突然降低,而后变化呈现一定的周期性;随着滑动的逐渐进行,运动体与基体表面的温度逐渐增加而后趋于稳定,两表面存在温度差;滑动后界面平均接触压力明显下降,而后趋于平缓。     

Tribological Performance of Coated Surfaces

The fundamentals of coating tribology is presented in a generalised holistic approach to friction and wear mechanisms of coated surfaces in dry sliding contacts. It is based on a classification of the tribological contact process into macromechanical, micromechanical, tribochemical contact mechanisms and material transfer. The tribological contact process is dominated by the macromechanical mechanisms, which have been systematically analysed by using four main parameters: the coating-to-substrate hardness relationship, the film thickness, the surface roughness and the debris in the contact. In this paper special attention is given to the microlevel mechanisms, and in particular new techniques for modelling the elastic, plastic and brittle behaviour of the surface by finite element (FEM) computer simulations. The contact condition with a sphere sliding over a plate coated with a very thin hard coating is analysed. A three dimensional FEM model has been developed for calculating the first principal stress distribution in the scratch tester contact of a diamond spherical tip moving with increased load on a 2μm thick titanium nitride (TIN) coated steel surface. The model is comprehensive in that sense that it considers elastic, plastic and fracture behaviour of the contact surfaces. By identifying from a scratch experiment the location of the first crack and using this as input data can the fracture toughness of the coating be determined.     

Investigation of dry friction of submicrometer thick polyurethane films on a hard,smooth substrate

Our research on friction mechanisms has shown that the frictional force between sliding surfaces at moderate speeds and loads is primarily due to mechanical effects, such as plowing of surfaces by entrapped wear particles. In the absence of mechanical interactions, the coefficient of friction is extremely low. In this paper, one way of eliminating mechanical interactions is presented. Hard, smooth single crystal silicon wafers were dip coated in aqueous polyurethane dispersions. The film thickness was varied from 90 to 250 nm by diluting the elastomeric material with different amounts of water, yielding a mean surface roughness comparable with that of the substrate, i.e. 3 nm. Borosilicate glass balls, 4 mm in diameter, were used as the counterface. An ultra-low friction coefficient of 0.04 in dry sliding was achieved through an appropriate selection of film thickness and crosslinking density. The friction coefficient of such films is dependent on the normal load, exhibiting a minimum friction coefficient under a specific normal load. Higher loads result in tearing of the surface, whereas at low loads the viscoelastic losses of the surface layer contribute to the increase in the coefficient of friction. Under the optimal normal load and film thickness, there is no visible damage to the surface after the friction tests. However, atomic force microscopy of the tested surfaces revealed that, even when the friction coefficient was as low as 0.05, the surface of the films was plowed. In general, a higher friction coefficient corresponds to greater surface damage.     

Influence of surface roughness of PVD coatings on tribological performance in sliding contacts

The influence of surface roughness on the tribological performance, i.e. friction, wear and material pick-up tendency, of two different commercial PVD coatings, TiN and WC/C, in sliding contact with ball bearing steel has been evaluated using two different types of sliding wear laboratory tests. Post-test characterisation using SEM/EDS, AES, ToF-SIMS and XPS was used to evaluate the prevailing friction and wear. The results show that the surface roughness of the coating is of importance in order to control the initial material pick-up tendency and thus the friction characteristics in a sliding contact. Once initiated, the material pick-up tendency will increase, generating a tribofilm at the sliding interface. For steel-TiN sliding couples a FeO-based tribofilm is generated on the two surfaces and FeO/FeO becomes the sliding interface (interfilm sliding) resulting in a high friction coefficient. For steel-WC/C sliding couples the WC/C displays a pronounced running-in behaviour which generates a WO₃-based tribofilm on the steel surface while a carbon rich surface layer is formed on the WC/C surface, i.e. WO₃/C becomes the sliding interface (interface sliding) resulting in a low friction coefficient.     

棉纤维束与金属摩擦行为研究

目的 基于接触力学模型对棉纤维与不锈钢303摩擦辊摩擦行为进行研究。方法 采用自制绞盘式摩擦试验装置,从预加张力、粗糙度、摩擦速率和棉纤维束包角4个方面探究了棉纤维束与金属摩擦辊表面的摩擦行为,并建立棉纤维与粗糙峰接触模型对试验结果加以验证。结果 预加张力与摩擦力呈正相关,与摩擦因数呈负相关。摩擦力与摩擦因数随所选粗糙度的增大而减小。摩擦速率只对摩擦系统达到稳定的周期数有影响,对摩擦行为的影响较小,摩擦速率较小时,系统达到稳定所需周期更长。包络角度增大,摩擦力增大,摩擦因数变化较小,包络角度越小震荡越明显。结论 与摩擦速率及包络角度相比,粗糙度和预加张力对棉纤维束与金属表面摩擦磨损行为影响更大。     

Contact behaviour of the original and substituted real surfaces

Several engineering components transfer load under sliding contact. Between two sliding bodies there are contacts over only small areas of the nominal contact area due to the surface roughness and surface waviness. The nominal contact pressure is therefore much smaller than the real contact pressure acting over the real contact areas inside the contour contact areas.To evaluate the contact parameters (location of the real contact area and contact pressure distribution) different numerical methods have been used in the past decades. The latest ones can consider the real surfaces based on measured surface roughness data.The present study focuses on the characteristic features of the asperities using hemispherical, ellipsoidal and parabolic surfaces as substituting surfaces. In each case the location of the real contact area and contact pressure distribution are evaluated and compared with the results representing the original measured surfaces. The best agreement was obtained if paraboloides were used for substituting asperities.This technique can provide statistical type results characterising the contact behaviour, if the substituting asperities are considered.     

不锈钢基板表面粗糙度对水膜吸附夹持的影响

目的从水膜吸附的功能研究出发,对不锈钢基板表面进行不同粗糙度处理,并测量不同基板对蓝宝石晶片的吸附力以及切向摩擦力的影响,从而得出粗糙度对水膜吸附效果的影响规律。方法通过砂纸打磨、研磨、抛光等方法,得到不同平均粗糙度(Sa=633.4、332.6、116.2、64.5、41.4nm)的不锈钢基板。利用接触角计对液滴在基板表面形成的静态接触角进行拍摄,得出不同粗糙度不锈钢表面的润湿性能。开发设计高精度的多维力测量平台,测量蓝宝石晶片在不同粗糙度不锈钢基板上润湿后的吸附力和切向摩擦力,并与未润湿的基板得到的测量结果进行对比研究,得出粗糙度对吸附力和摩擦系数的影响规律。结果5种粗糙度的不锈钢基板的静态接触角均小于90°,属于亲水性材料。水膜吸附条件下,吸附力大小随粗糙度的增加而减小;接触角大小随粗糙度的增加而增大,且吸附力的减小率和接触角的增大率趋势相似;摩擦系数随粗糙度的增大而增大。基板表面粗糙度较大时,水膜提供一定的粘滞力,使水膜吸附条件下比无水膜时的切向摩擦力更大;当粗糙度较小时,水膜更多的是润滑作用,此时比无水膜时的切向摩擦力要小得多。结论基板表面粗糙度较小时,基板能提供较大的吸附力,而摩擦力不如无水膜时的大;基板粗糙度较大时,吸附力相对较弱,但是摩擦力比无水膜的更大。在选择不锈钢基板作为水膜吸附夹持基板时,在保证足够吸附力的条件下,可以适当提高基板的表面粗糙度,抵抗晶片抛光过程中受到抛光垫的摩擦力。     

滑动摩擦接触对母线弯曲成形质量的影响

目的 研究滑动摩擦接触对1060纯铝母线弯曲成形质量的影响,得到表面质量更好的工件,降低废品率。方法 采用自行设计的V形三点式自由弯曲成形的摩擦力测试装置,通过更换不同表面粗糙度的凹模圆角、不同润滑介质以改变接触状况,进行一系列摩擦试验。通过钨灯丝扫描显微镜获得板料弯曲件表面微观形貌图,通过MATLAB软件对所采集的数据进行曲线处理。结果 得到不同粗糙度的凹模圆角以及不同润滑介质条件下的弯曲力-行程曲线。经测定,凹模圆角表面光滑时,无润滑状态下,最大摩擦力约为440 N;采用聚乙烯薄膜作为润滑介质,最大摩擦力约为100 N;采用聚四氟乙烯薄膜作为润滑介质时,最大摩擦力约为20 N。凹模圆角表面粗糙时,无润滑状态下,最大摩擦力约为235 N;采用聚四氟乙烯薄膜作为润滑介质时,最大摩擦力约为28 N。结论 添加润滑介质可以有效降低板料与凹模圆角之间摩擦力大小,进而提高弯曲件表面成形质量。滑动摩擦条件下,无论光滑还是粗糙的凹模圆角,采取润滑措施均能有效提高弯曲成形工件的表面质量,且聚四氟乙烯薄膜作为润滑介质时,得到的板料表面质量最好。     

Influence of tool roughness and lubrication on contact conditions in skin-pass rolling

The special contact conditions in skin-pass rolling of steel strip are examined by experimental as well as numerical analysis studying plane strain upsetting of thin sheet with low reduction applying long narrow tools with smooth and roughened surfaces under dry friction and lubricated conditions. The influence of friction on the extent of a central sticking region is determined by an elasto-plastic FEM analysis of the plane strain upsetting. The experimental results obtained by measuring the local surface extension using markers made by Micro Vickers indentation verify the FE analysis and show significant influence of tool roughness and lubrication on the contact conditions for varying pressure. The central sticking region was larger for larger friction or tool roughness. At increasing pressure a sudden change in deformation pattern appeared with drastic elongation and sliding in case of lubrication. This deformation pattern is also affected by the tool roughness.     

Analytical and experimental investigation of rake contact and friction behavior in metal cutting

In this study, the friction behavior in metal cutting operations is analyzed using a thermomechanical cutting process model that represents the contact on the rake face by sticking and sliding regions. The relationship between the sliding and the overall, i.e. apparent, friction coefficients are analyzed quantitatively, and verified experimentally. The sliding friction coefficient is identified for different workpiece–tool couples using cutting and non-cutting tests. In addition, the effect of the total, sticking and sliding contact lengths on the cutting mechanics is investigated. The effects of cutting conditions on the friction coefficients and contact lengths are analyzed. It is shown that the total contact length on the rake face is 3–5 times the feed rate. It is observed that the length of the sliding contact strongly depends on the cutting speed. For high cutting speeds the contact is mainly sliding whereas the sticking zone can be up to 30% of the total contact at low speeds. From the model predictions and measurements it can be concluded that the sticking contact length is less than 15% for most practical operations. Furthermore, it is also demonstrated that the true representation of the friction behavior in metal cutting operations should involve both sticking and sliding regions on the rake face for accurate predictions. Although the main findings of this study have been observed before, the main contribution of the current work is the quantitative analysis using an analytical model. Therefore, the results presented in this study can help to understand and model the friction in metal cutting.     

不同接触尺度下石墨烯摩擦学性能研究进展

石墨烯是石墨的基本结构组成单元,是由碳原子以sp^2杂化轨道组成、呈六角型蜂巢状且具有独特二维结构的纳米材料。由于其优异的机械及润滑特性,成为近年来国内外摩擦学领域研究的聚点和热点。大量研究表明:石墨烯不仅在微纳接触尺度下展现出超润滑性能,而且在宏观接触方式下也展现出非凡的摩擦学特性。详细介绍了石墨烯在微观接触和宏观接触状态下摩擦学性能的影响因素。在微观接触状态下,滑动发生在完美、平滑的基础面晶面上,施加的载荷小,接触面小,其摩擦学性能主要受到结构形变、表面清洁(包括缺陷、环境、化学官能团)、公度/非公度结构等因素影响。在宏观接触状态下,施加的载荷大,接触面积大,影响因素众多,除了上述在微观接触状态时提到的影响因素,还涉及材料本身存在的各向异性、晶界以及宏观力的破坏等作用,受到棱边键及摩擦引起的悬键暴露和定序摩擦界面的形成的影响。基于此,分析了石墨烯获得不同尺度超低摩擦所需的必要条件和润滑机制,展望了石墨烯摩擦学研究未来发展方向和仍需解决的问题。     

Polishing of polycrystalline diamond by the technique of dynamic friction, part 1: Prediction of the interface temperature rise

This paper investigates the interface temperature rise in polishing a polycrystalline diamond (PCD) surface. First, the Greenwood–Williamson's statistical asperity model is applied to characterise the surface roughness of a PCD specimen. The result is then used to estimate the contact area and total number of contact asperities under an applied polishing load. The heat generated is taken as the product of the friction force and the relative sliding velocity between the PCD asperities and the metal disk surface. The Jaeger's moving heat source analysis is then applied to determine the fractions of heat flux flowing into the PCD asperities and their counterpart in contact sliding and to give rise to the average temperature rise. A comparison with the observations made in the authors' experiments and those reported in the literature showed that the model predicts very well the temperature rise at the polishing interface.     

考虑表面形貌的接触应力分析

目的 研究规则和实际表面形貌对二维平面线接触模型的影响.方法 基于线接触几何特性,考虑表面受载后的弹性变形特性,结合规则和实际粗糙表面的形貌特征,并考虑磨合对表面的影响,采用移动平均滤波方法对实际粗糙表面进行光滑处理,利用共轭梯度法,求解表面接触压力和摩擦力,计算二维平面内接触近场应力分布,同时采用修正的离散卷积快速傅里叶变换方法提高计算效率.对比验证二维接触模型的准确性,并对比规则和实际粗糙形貌表面受法向载荷和切向载荷时平面应力分布云图中的应力分布和大小.结果 圆面与圆形微凸体接触时,圆形凸体半径越小,最大Mises应力值越大,半径增大,最大Mises应力减小.多个圆形微凸体及正弦微凸体对应力分布影响类似.当考虑切向力时,会对应力分布云图中的应力分布形状和大小产生巨大影响.在相同平面表面形貌时,摩擦系数越大,Mises应力越大,并且应力沿摩擦力方向发生偏移.结论 数值模型能准确计算出粗糙平面的应力.当圆面与规则表面轮廓平面接触时,与光滑表面接触时相比,Mises应力分布呈现很大不同,微凸体越小,应力越集中;摩擦力会使接触压力和近场Mises应力产生偏移,摩擦系数越大,偏移越明显.当考虑实际粗糙表面时,在粗糙界面尖端接触区域产生应力集中.经过磨合表面采用平均滤波光滑处理后,粗糙界面尖端接触区域应力集中将大幅减小.     

初始表面粗糙度对沟槽织构摩擦性能的影响

目的探究初始表面粗糙度大小对激光沟槽织构化表面摩擦性能的影响规律。方法采用脉冲光纤激光器在不同粗糙度的45#钢试样表面制备具有不同深度、规则排列的沟槽织构,利用摩擦磨损试验机进行销-盘式往复摩擦试验,研究初始表面粗糙度对沟槽织构化表面摩擦系数的影响规律,以及不同初始表面粗糙度条件下,激光沟槽织构化表面的摩擦学行为。结果沟槽织构能够有效降低表面的摩擦系数,初始表面粗糙度、载荷和速度的大小对沟槽织构的润滑减摩性能有较大影响。在较低的载荷下,沟槽织构能有效提高表面的流体动压润滑效应;在较高的载荷下,沟槽织构能够有效改善表面的边界润滑性能。存在最优初始表面粗糙度,使得摩擦系数达到最小,初始表面粗糙度最优值的大小应根据载荷和速度大小来确定。结论根据摩擦副表面的载荷和速度工况条件,合理优化初始表面粗糙度能使沟槽织构获得较为理想的润滑减摩效果。     

金属塑性成形中摩擦模型的研究进展

详细论述了当前有关塑性成形摩擦建模现状的几点研究展望。摩擦是塑性成形中一项重要且复杂的边界条件,直接影响成形过程。鉴于目前塑性成形摩擦模型的发展状况,将它们分为经典摩擦模型、基于粘附理论的摩擦模型、引入相对滑动速度的摩擦模型、考虑表面粗糙度的摩擦模型和基于专用测试装置的摩擦模型等五类进行总结,并对各模型的构成、特点和适用范围进行了较为详细的论述。经典摩擦模型由于经典理论的自身不足,很难准确描述塑性成形中的摩擦行为;粘附理论从微观层面更好地解释了摩擦现象,General摩擦模型改善了摩擦模型适用性,但植入有限元程序较为困难;反正切修正提高了摩擦模型的可植入性,IFUM摩擦模型较为真实地反映了相对滑动速度的影响,这类模型已经作为商业化软件中的可选模型;与基于粘附理论的摩擦模型相比,考虑表面粗糙度的摩擦模型更细致地描述了真实接触表面对摩擦的影响。     

The Effect of Textured Surfaces with Different Roughness Structures on the Tribological Properties of Al Alloy

In this study, four kinds of roughness structures were constructed on the Al alloy surface using laser marking technology. The tribological properties of the surfaces under the condition of dry friction were carefully investigated. The results indicate that the fabricated surfaces have similar compositions. The hardness of surfaces can be improved after the laser surface treatment. Besides, the texturing of surfaces can efficiently reduce friction and improve friction resistance. However, the friction-reducing mechanisms are not the same. The surfaces with lined and grating grooves can remove wear debris away from the interfaces between steel balls and surfaces, while those with irregular protrusions and micro-orifices array would be able to trap wear debris in the microstructure. Furthermore, due to the different friction mechanisms of distinct roughness structures, their friction-reducing performances are greatly affected by the actual friction conditions (sliding speed and load), which offers a guide for constructing a specific roughness structure on the Al alloy surface to improve its friction resistance efficiently.