双粗糙表面滑动过程不同条件对Z向速度的影响分析

采用W-M函数建立具有分形特征的三维双粗糙面接触模型,在滑动速度、法向载荷及界面剪切强度等各种因素的相互作用下,动态探讨了粗糙体在滑动过程中的摩擦磨损情况。运用有限元方法对滑动过程进行模拟仿真,对Z向速度在各种因素作用下的变化规律进行分析,结果显示滑动速度较小情况下,Z向速度振幅较小;较小法向载荷情况下,Z向速度的振幅较大;界面剪切强度较小情况下,Z向速度的振幅较大。从功率谱看,较小界面剪切强度下摩擦振动变化比较强烈,而相应的所需要能量比较小。将这些结果与相关文献或实验进行比较,得出模拟仿真的合理性,同时通过研究法向载荷、滑动速度、界面剪切强度等工况参数对摩擦振动的影响,以期为摩擦学设计和摩擦材料的制备提供理论参考。     

滑动摩擦过程中接触力的研究

在接触面上接触力对材料的破坏起着决定性作用,有必要研究接触力的变化情况。研究了滑动摩擦过程中,不同的界面剪切强度、不同的法向载荷与不同滑动速度对粗糙面的接触力影响,研究表明随着界面剪切强度增加,粗糙表面间有向上的拉力,随着法向载荷的增大,平均切向接触力的增长速率加快。在润滑条件相同时,滑动速度存在最佳值使平均滑动阻力最小。     

粘着与速度对滑动摩擦变形影响的研究

研究了在不同剪切强度和滑动速度参数下,两粗糙面间滑动过程中对等效塑性应变、y向和x向变形的影响。随着界面剪切强度增加,等效塑性应变的分布由良好对称转变成沿接触界面与滑动反方向扩大。在滑动中,界面剪切强度和滑动速度都影响y向和x向变形。随着滑动速度的减小,粗糙表面的变形越大黏着效应越明显。     

面接触边界润滑模型研究

根据经典的热传导公式,求解面接触边界润滑条件下的接触温度,再根据吸附热和摩擦因数、接触温度之间的关系建立一种求解边界润滑摩擦因数的模型。利用模型计算一个实例,通过分析发现,在载荷较低时,摩擦因数随滑动速度或载荷的变化变动很小,载荷较高时,摩擦因数随滑动速度或载荷的增大而增加;摩擦因数随接触温度的增加而增加;相对油亏量和摩擦因数有着基本一致的变化规律。通过在自制滑块-摩擦盘机构上的实验证实,实验结果和理论结果有相似的变化规律。     

二维粗糙表面滑动摩擦的研究

能源接近一半消耗在摩擦上,摩擦引起的磨损为主要的机械零件失效方式,通过对摩擦和磨损机理的研究能有效地延长机械零部件的使用寿命。通过对建立的二维双粗糙弹塑性分形模型的实例分析,分析界面剪切强度和法向载荷对磨损率和平均摩擦系数的影响。界面剪切强度增加,磨损率增加较快。随着界面剪切强度的增加平均摩擦系数增加;当界面剪切强度较小时,随着法向载荷的增加平均摩擦系数增加,当界面剪切强度较大时,随着法向载荷的增大,平均摩擦系数先减小后又逐渐增大。     

一种点接触粗糙表面摩擦行为的预估方法研究

针对润滑状态下连接界面的接触问题,对流体动力油膜和粗糙表面的摩擦特性进行了研究,提出了一种点接触粗糙表面摩擦行为的预估方法。首先,基于载荷分配思想建立了粗糙表面摩擦模型,利用Hertz理论中的最大接触压力分别确定了微凸体高度分布服从高斯分布、指数分布以及三角分布时粗糙表面的接触载荷;然后,通过弹性流体动力润滑膜厚公式求解了流体动力油膜承担的载荷;最后,绘制了滑动速度-摩擦系数曲线,模拟了整个润滑过程中连接界面摩擦系数的变化。研究结果表明:仿真结果与试验数据具有一致性,且不同法向载荷、粗糙表面形貌、润滑剂粘度以及假设的微凸体高度分布对摩擦系数的影响程度不同;微凸体假设为高斯分布时,仿真结果更接近试验数据;该方法可以为机械结构的润滑状态预测提供理论基础。     

双粗糙接触模型的表面温度在滑动摩擦过程中变化分析

建立三维双粗糙体分形表面的热力耦合接触模型,在固定滑动速度工况下综合考虑了钛合金材料的磨损失效、界面粘着及接触过程中的热力耦合,动态探讨了粗糙体在滑动过程中接触表面的温度变化情况。运用有限元方法对滑动过程的温度场进行模拟仿真并得出:滑动摩擦初始时刻摩擦表面接触温度急剧上升,随着滑动距离的增加,最高接触温度处于波动状态;界面剪切强度越大,最高温度越高。通过研究接触表面的温度场分布情况,以探索滑动过程钛合金材料摩擦磨损的真正起因。将结果与相关文献实验进行比较,得出了模拟仿真的合理性。     

大滑滚比工况下弹流摩擦试验研究

针对大滑滚比工况,在自行改造的双盘摩擦磨损试验机上,完成了一系列线接触弹流摩擦试验;测量了不同工况下的摩擦因数,对试验结果进行了全面的分析,得到了一系列摩擦曲线,反映了多种具体工况下,各工况参数对摩擦因数的影响.结果表明:载荷比较小时,摩擦因数对载荷或转速的变化非常敏感;随着载荷的增大,摩擦因数对载荷与转速的敏感程度急剧下降;大滑滚比工况下,摩擦因数随滑滚比的变化幅度不大;摩擦因数随载荷增大而减小,且减小幅度随载荷增大而减小;摩擦因数随转速增大而减小,且减小幅度随转速增大而减小.     

二维粗糙面间滑动摩擦过程中平稳性研究

基于现有的分形模型,考虑到黏着现象的普遍性,通过在界面引入黏着剪切强度并考虑材料断裂,建立了考虑黏着、弹塑性变形影响的二维分形金属粗糙面间的滑动摩擦模型,并运用有限元软件进行数值求解.通过设置不同界面剪切强度及滑动速度参数,研究了两粗糙面间滑动过程中的平稳性.对建立的二维双粗糙弹塑性分形模型的实例进行分析,结果表明:当界面剪切强度较小时,法向速度波动幅值增大;当界面剪切强度增大到一定值后,法向速度波动幅值迅速的减小,使滑动较为平稳.滑动速度对平稳性的影响较大,滑动速度越大,滑动过程中振动越剧烈.     

冰面上橡胶滑动摩擦特性的数值分析

基于摩擦熔化理论,考虑流体动力润滑和热传导的耦合作用,采用打靶法和线性插值法获得了冰面上滑动橡胶块摩擦界面上的压力、流体膜厚及温度的无量纲分布规律,研究了胶块的摩擦特性。结果表明,摩擦界面上的压力峰值随载荷的减小向界面中部移动,摩擦因数与滑动速度的平方根成正比,而与载荷和试样长度的平方根成反比。所用方法可应用于其它材料在冰面上的摩擦及其它情况下的摩擦熔化问题。     

Dependence of polymer sliding friction on normal load and contact pressure

The sliding friction of bulk polymers was studied varying the normal load, contact pressure and sliding velocity. The variation of the area of apparent contact A with normal load W was also measured both under the sliding and unloaded conditions. For the sliding condition A ∝ W, while for the unloaded condition A ∝ Wⁿ where n is less than unity. The friction measurements were performed on a tribometer in the low load range and on a lathe using a strain gage dynamometer in the high load range. It was found that the coefficient of friction depends upon the velocity and pressure and the variation can be explained by the adhesion theory of friction in the light of the conditions at the interface. The measurement of sliding friction in an extrusion process shows that the coefficient of friction decreases with contact pressure and the interface friction shear stress is almost equal to the bulk shear strength of the material. All of these findings support the adhesion theory of friction for polymeric materials.     

双粗糙面滑动过程考虑黏着时的摩擦学性能分析

建立了二维双粗糙体分形表面的接触模型,在固定滑动速度工况下考虑材料的磨损失效,针对是否考虑接触过程中的黏着因素,动态探讨了粗糙体在滑动过程中的摩擦磨损变化情况。运用有限元方法对滑动过程的摩擦磨损进行模拟仿真,得出考虑黏着因素的界面剪切强度τ=σy/3(σy为材料的屈服应力)时的摩擦因数平均值为0.48;对滑动过程是否考虑黏着因素的磨损率及振动情况进行分析比较,引入快速傅里叶函数对摩擦振动进行变换得到功率谱,结果发现,考虑黏着因素的情况下,相应的磨损率较大,功率谱低频成分较多,振动相对比较平缓,所需要的能量也相应比较大。将模拟仿真结果与实验进行比较,验证了模拟仿真的合理性,也加深了对摩擦磨损过程物理图像的理解。     

Effects of Interfacial Strength and Roughness on the Static Friction Coefficient

A finite element model is used to simulate sliding inception of a rigid flat on a deformable sphere under combined normal and tangential loading. Sliding inception is treated as the loss of tangential contact stiffness under combined effects of plasticity, crack propagation and interfacial slip. Energy dissipation distribution is used to quantify the relative contribution of these mechanisms on the increased compliance during tangential loading. Materials with different strength and toughness properties, and varying local interface conditions ranging from fully adhered to finite friction, are studied to relate variations in plastic deformations, crack and slip to the sliding inception. For fully adhered contact condition, crack and fracture toughness have no effect on sliding inception, with plasticity, the dominant failure mechanism. A measure of recoverable strain (yield strength to Young’s modulus ratio) is found to be the most influential parameter in sliding inception. Interfacial slip is expectedly the dominant mechanism for sliding inception for lower coefficient of friction, modeling lubricated contacts. Interplay of plasticity and interfacial slip is found to govern the onset of sliding for higher local friction coefficients. Furthermore, the single asperity results are incorporated in a statistical model for nominally flat contacting rough surfaces under combined normal and tangential loading to investigate the stochastic effects due to surface roughness and material property uncertainties. The results show that the static coefficient of friction strongly depends on the normal load, material properties, local interfacial strength and roughness parameters.     

Effects of Normal Load,Sliding Speed,and Surface Roughness on Tribological Properties of Niobium under Dry and Wet Conditions

The effects of normal load, sliding speed, and surface roughness on the friction and wear of high-purity niobium (Nb) during sliding without and with an introduction of water were systematically investigated. Increasing the normal load or sliding speed decreased the friction of the Nb under the both dry and wet conditions because the increased wear of the Nb decreased the interfacial shear strength between the steel ball and Nb by promoting the surface roughening and the production of wear debris. However, the Nb tested at the lowest sliding speed under the lowest normal load with water exhibited the lowest friction and wear due to the formation of oxide layer on the wear track. The friction and wear of the Nb tested under the dry condition decreased with increased surface roughness because the higher interfacial shear strength between the steel ball and smoother Nb resulted in the earlier breakdown of the native oxide layer and direct contact between the steel ball and Nb. However, increasing the surface roughness of the Nb increased its friction and wear under wet conditions, probably due to the easier breakdown of the oxide layer that formed on the rougher surface during sliding. The tribological results clearly showed that the introduction of water during sliding had a significant influence on the tribological properties of the Nb.     

Numerical and analytical modeling of orthogonal cutting: The link between local variables and global contact characteristics

The response of the tool-chip interface is characterized in the orthogonal cutting process by numerical and analytical means and compared to experimental results. We study the link between local parameters (chip temperature, sliding friction coefficient, tool geometry) and overall friction characteristics depicting the global response of the tool-chip interface. Sticking and sliding contact regimes are described.The overall friction characteristics of the tool are represented by two quantities: (i) the mean friction coefficient qualifies the global response of the tool rake face (tool edge excluded) and (ii) the apparent friction coefficient reflects the overall response of the entire tool face, the effect of the edge radius being included. When sticking contact is dominant the mean friction coefficient is shown to be essentially the ratio of the average shear flow stress along the sticking zone by the average normal stress along the contact zone. The dependence of overall friction characteristics is analyzed with respect to tool geometry and cutting conditions. The differences between mean friction and apparent friction are quantified. It is demonstrated that the evolutions of the apparent and of the mean friction coefficients are essentially controlled by thermal effects. Constitutive relationships are proposed which depict the overall friction characteristics as functions of the maximum chip temperature along the rake face. This approach offers a simple way for describing the effect of cutting conditions on the tool-chip interface response. Finally, the contact length and contact forces are analyzed. Throughout the paper, the consistency between numerical, analytical and experimental results is systematically checked.     

Friction and wear behavior of flame-sprayed PEEK coatings

Polyetheretherketone (PEEK) becomes of great interest to applications as bearing and slider materials. In this paper, PEEK coatings with three kinds of crystallinities were deposited using the flame spray process. Employing a uniform design experiment, the friction and wear behavior of the three PEEK coatings were systematically investigated under dry sliding conditions against a 100C6 counterbody on a ball-on-disc arrangement for several loads and sliding velocities. For the three coatings, the friction coefficient significantly followed the normal distribution. The average friction coefficients appeared to decrease while increasing the sliding velocity, but were insensitive to the applied load in the range of investigation. Among the three coatings, the higher the crystallinity of the coating, the lower its average friction coefficient was. The wear rate of the coating with the lowest crystallinity decreased with an increase in the load and a decrease in the sliding velocity. The wear rate of the coating with the intermediate crystallinity decreased with an increase in the load, but increased with an increase in the sliding velocity at lower loads, and then decreased with an increase in the velocity at higher loads. The wear rate of the coating with the highest crystallinity decreased with the increase of both the load and the sliding velocity. The wear mechanisms of the different coatings are explained in terms of plastic deformation, plogh marks and fatigue tearing.     

铜碲硒铁合金的干摩擦磨损性能

利用环-盘式摩擦磨损试验机研究了铜碲硒铁合金的干摩擦磨损行为,分析了载荷和摩擦速度等参数对该合金摩擦磨损性能的影响,并用扫描电子显微镜对磨损形貌进行了观察.结果表明:铜碲硒铁合金的摩擦因数随载荷的增加变化不大,但随摩擦速度的增加而明显增大;合金的磨损率随载荷和摩擦速度的增加均增大;在轻载低速条件下,合金的磨损机制以犁削磨损和粘着磨损为主;在重载高速条件下,磨粒磨损和粘着磨损加剧.     

Experimental study of air squeeze effect on high-frequency friction contact

A special tribometer was developed which was used to test sliding friction force between PTFE-based composites and bronze with normal (out-of-plane) or transverse (in-plane) high-frequency vibrations under three different environmental pressures. The influences of environmental pressure, vibration amplitude and sliding velocity on sliding friction coefficient were studied. The results show that the effect of environmental pressure on reduction of sliding friction is outstanding. With the increase of vacuum, the reduction of sliding friction by high-frequency vibrations decreased, especially the reduction of sliding friction by normal vibration. The sliding friction coefficient with high-frequency vibrations slowed down as the vibration amplitude increased. With increase of sliding velocity, the time-averaged friction coefficient with transverse vibration increased.     

In Situ Analysis of Third Body Contributions to Sliding Friction of a Pb–Mo–S Coating in Dry and Humid Air

Reciprocating sliding tests of ion-beam deposited (IBD) Pb–Mo–S coatings were performed with an in situ tribometer that allows real-time visualization and Raman analysis of the sliding contact through a transparent hemisphere. Experiments were performed in dry air, ambient air (∼50% RH) and mixtures of dry and humid air cycled between low and high humidity. Third bodies formed in the sliding contact were monitored through an optical microscope and analyzed by Raman Spectroscopy. Third body velocity accommodation modes were identified and correlated with friction behavior in dry and ambient air. The dominant velocity accommodation mode in both dry and humid air was interfacial sliding between the outer surface of the transfer film and the wear track; this interface, based on present and earlier studies, is crystalline MoS₂. Therefore, the friction coefficient was controlled by the interfacial shear strength of MoS₂ sliding against MoS₂. Humid air sliding was accompanied by a rise in the friction coefficient and a small but observable second velocity accommodation mode: shear/extrusion of the transfer film. It is concluded that the friction rise in humid air was due to an increase in the interfacial shear strength, and that the rise in friction caused the third body to deform rather than the deformation causing the friction to rise.     

铜镍锡合金干滑动磨损行为研究*

大型装备上的铜镍锡合金工件常常工作在润滑不足或者无润滑的工况条件下,因此有必要研究铜镍锡材料在干摩擦下的性能。采用CEF-I型销-盘式磨损试验机对铜镍锡合金在不同载荷与转速条件下进行干滑动摩擦磨损试验,采用扫面电子显微镜、能谱仪和轮廓检测仪对磨损表面进行检测,研究铜镍锡合金的干滑动磨损行为,并探讨其磨损机制。结果表明：在转速一定时,平均摩擦因数随载荷增大总体呈波动下降趋势,铜镍锡合金试件磨损量先缓慢增加,在较高载荷下快速增加;在载荷一定时,平均摩擦因数随转速增大先增大后减小,铜镍锡合金试件磨损量总体呈先减小后增大趋势;随着载荷的增大,铜镍锡合金的磨损机制由轻微黏着和剥层磨损到明显的黏着和剥层磨损,再到严重的黏着和疲劳磨损,随着转速的增大,铜镍锡合金的磨损机制由磨粒磨损转变为黏着磨损再到剥层磨损。在较低载荷下,随着转速的增大,铜镍锡合金摩擦磨损性能保持很好的稳定性,因此铜镍锡合金适用在高转速较低载荷的干摩擦工况下。