考虑边界膜强度的滑动摩擦副混合润滑模型研究

针对边界膜对摩擦副润滑状态的影响，提出一种能够综合反映压力及剪切速率对边界膜失效综合影响的边界膜强度模型，并基于润滑状态测试结果通过拟合获得模型参数；将该边界膜强度模型与流体动压润滑模型、粗糙表面接触模型耦合，建立考虑边界膜强度的混合润滑模型，并通过轴瓦摩擦实验机润滑测试结果对模拟结果进行验证。和现有典型混合润滑模型相比较，该混合润滑模型可以更准确地反映摩擦副的实际润滑状态以及摩擦因数变化规律。运用考虑边界膜强度的混合润滑模型分析轴瓦零件润滑状态转化特性和机制。结果表明：在存在边界润滑的混合润滑条件下，当加载力小于临界载荷，边界膜几乎未发生破裂，摩擦因数随载荷增加缓慢变大，其数值均较小；当加载力加至临界载荷，边界膜破裂，摩擦副微凸体接触区域出现干摩擦，摩擦因数出现突然增加，表明该摩擦副由边界润滑为主的混合润滑状态过渡到以干摩擦为主的润滑状态。     

摩擦热和夹杂对高速微型球轴承接触亚表面裂纹萌生的影响

为探究高速工况下微型球轴承摩擦生热和材料夹杂对接触亚表面裂纹萌生的影响，基于赫兹接触理论与内聚力单元的本构关系建立了微观接触数值计算模型，设置具有热力耦合的自由单元，引入摩擦热和夹杂，研究在联合载荷作用下温升和夹杂对接触亚表面裂纹萌生的影响。结果表明：夹杂与温升均会促进裂纹的萌生且材料较硬的夹杂更易萌生裂纹；夹杂边界出现应力集中，摩擦生热使夹杂边界更易形成裂纹且裂纹大多在夹杂的左右两端。通过轴承温升的宏观试验结果与微观仿真结果对比验证了模型的正确性。     

Ag膜在干摩擦、油和脂润滑下的摩擦学性能研究

精密运转部件表面沉积一层软金属银和银基固体薄膜可以有效地降低摩擦、减小磨损。通过钢球/镀Ag膜摩擦盘在干摩擦、4122油和L252脂润滑条件下的球-盘摩擦学试验,研究Ag膜在油和脂复合润滑下的摩擦学性能,分析润滑条件、载荷、速度对Ag膜摩擦因数的影响。试验结果表明:在4 N法向载荷和油、脂润滑下,与干摩擦相比,镀Ag膜摩擦副的最大静摩擦因数分别减小了10.7%和6.1%;在0～2 000 r/min转速范围内,Ag膜摩擦因数随转速增加而减小,与干摩擦相比,油润滑下Ag膜摩擦因数减小9%～48%,脂润滑下Ag膜摩擦因数减小17%～52%。Ag膜在干摩擦、4122润滑油和L252润滑脂复合润滑下,摩擦因数均随载荷增加而降低;Ag膜摩擦副/钢球在油、脂复合润滑下启动摩擦力矩小,摩擦副在宽转速范围内摩擦因数变化小,运转平稳。     

三维压电介质中不同电边界条件的裂纹分析

在不可导通、可导通和半可导通等三种电边界条件下，系统研究并给出三维无限横观各向同性压电介质中平行于各向同性面的、任意形状的平片裂纹在任意载荷作用下的不连续位移和不连续电势边界积分方程方法。无论平片裂纹的形状如何，结果都表明：对不可导通裂纹，应力强度因子只与机械载荷有关，电位移强度因子只与电载荷有关；而可导通裂纹的应力强度因子和电位移强度因子只与机械载荷有关，电位移强度因子由机械载荷通过压电效应产生。半可导通裂纹所对应的边界积分方程组为非线性方程组，文中给出一种迭代解法。在均布的机械载荷和电载荷作用下。裂纹腔内的电位移为一常数，只与外加载荷有关，而与裂纹形状无关。     

钢与Si3N4摩擦副中边界膜生成机理的探讨

边界摩擦是摩擦副经常出现的一种状态,论述了钢与Ｓｉ３Ｎ４（氮化硅）摩擦副边界润滑机理,指出了在钢表面形成化学反应膜的过程及在Ｓｉ３Ｎ４表面能生成吸附膜,探讨了软金属沉积在Ｓｉ３Ｎ４表层及亚表层从而形成一层软金属润滑层的方法。     

电化学表面氧化处理的TC4在水基介质中的耐蚀耐磨行为

在销-盘式摩擦磨损试验机上,利用构建的三电极电化学测量分析系统,分别研究了在水、H2SO4、NaOH、NaCl溶液中进行静、动态电化学表面氧化处理后的TC4钛合金的耐蚀及耐磨性.结果表明,动态和静态条件下施加阳极处理电压都能使TC4表面产生氧化膜,氧化膜的耐蚀性与所加电压、载荷的大小有关;动态研磨过程中表面氧化膜生成与机械去除的过程是同时进行的;在水溶液中产生的氧化膜的耐腐蚀性最好.     

一种新型摩擦材料在水润滑下的摩擦学性能研究

采用环块式摩擦磨损实验研究了一种新型摩擦材料在水润滑状态下不同载荷与转速对试样摩擦学性能的影响,并对比干摩擦条件下的摩擦学性能变化,借助磨损表面形貌观察分析其磨损机理。实验结果表明:水润滑条件下,摩擦系数随着载荷的增大而减小,随着转速的提高先增加后减小;磨损率随着载荷与转速的提高都减小。相同载荷与转速下,干摩擦时磨损机理以磨粒磨损和黏着磨损为主,而水润滑条件下水形成边界润滑,磨损机理以磨粒磨损和轻微的黏着磨损为主;水润滑条件下摩擦系数和磨损率均低于干摩擦,主要是由于水起到了润滑和冷却的作用,阻止了转移膜的形成,并在材料表面形成水膜起到了边界润滑的作用。     

PTFE三层复合材料摩擦过程的界面动态迁移

在面接触摩擦磨损试验机上考察了PTFE三层复合材料的摩擦磨损性能,观察了其摩擦过程转移膜形成的动态过程,分析了其摩擦磨损机制。结果表明:PTFE三层复合材料在与对偶钢件摩擦时,会在摩擦界面上发生界面迁移,导致在对偶件表面形成一层转移膜。界面迁移的动态过程是:转移膜的形成开始是由于微切削机制使得PTFE复合材料镶嵌于对偶件表面的凹坑和微沟槽处,进而在机械和摩擦化学的作用下得以生长变大,最后形成较完整的转移膜。转移膜在形成与长大过程中,始终伴随着局部脱落以及再修复的过程,在形成较完整的转移膜后处于动态平衡。摩擦过程中转移膜的形成可以有效地稳定摩擦因数,减小磨损。     

碳/碳复合材料的载流摩擦磨损性能及机理

在高速载流摩擦磨损试验机上对碳/碳复合材料进行摩擦磨损试验,研究了不同电流、载荷和滑动速度下复合材料的摩擦因数、磨损率及磨损表面形貌,并分析了磨损机理。结果表明:在一定载荷作用下,随电流和滑动速度增大,碳/碳复合材料的摩擦磨损性能先保持良好而后趋于恶化;在电流和滑动速度一定的条件下,较低和较高的载荷都会恶化碳/碳复合材料的摩擦磨损性能;随着摩擦表面温度升高,碳/碳复合材料基体开始氧化流失,碳纤维脱落形成磨屑,从而导致磨粒磨损;随后摩擦表面的高温使磨屑软化,磨屑在机械应力作用下逐渐被碾压成碳膜,形成粘着磨损;磨损表面温度的进一步升高以及高速冲击的作用破坏了碳膜的完整性,从而恶化了碳/碳复合材料的摩擦磨损性能。     

机械合金化FeS/Cu复合材料的摩擦磨损性能

以FeS和CuSn8Ni1粉末为原料,利用机械合金化技术和粉末冶金技术制备了FeS/Cu复合材料,探讨了不同载荷情况下所制备的FeS/Cu复合材料的摩擦学性能及润滑膜与转移膜特征。结果表明：机械合金化提高了FeS与铜合金基体界面结合性能,进而提高了材料减摩耐磨性能;当载荷较小时,摩擦副表面接触不稳定,复合转移膜不连续,摩擦因数波动大;载荷较大时,复合转移膜易破损,材料的减摩耐磨性能变差;当载荷为150 N时,载荷适宜,材料表面软化,复合转移膜更加完整,摩擦因数较小。     

Improving Electrical Discharge Machined Surfaces Using Magnetic Abrasive Finishing

A recast layer is invariably present on surfaces produced by electrical discharge machining (EDM). For some metals with high hardness, the recast layer may contain micro-cracks. This damaged layer can affect the service life of the parts produced by this method. This investigation demonstrates that magnetic abrasive finishing (MAF) process using unbonded magnetic abrasives (UMA), can improve the quality of EDM machined surfaces effectively. The UMA used herein is a mechanical mixture of steel grit and SiC abrasive. SKD11 tool steel was used as the workpiece. Experimental results show that the recast layer and micro-cracks on EDM machined surfaces can be completely removed and a new surface of roughness on the order of 0.04 μm Ra can be produced. Additionally, experiments using the Taguchi method and L₁₈ orthogonal array enable the determination of the optimum process conditions for improving the surface finish. Further, the significance of the control factors was identified with the assistance of analysis of variance (ANOVA), and the optimum combination of the process parameters was verified by conducting several confirmatory experiments.     

Improving Electrical Discharge Machined Surfaces Using Magnetic Abrasive Finishing

Abstract

A recast layer is invariably present on surfaces produced by electrical discharge machining (EDM). For some metals with high hardness, the recast layer may contain micro-cracks. This damaged layer can affect the service life of the parts produced by this method. This investigation demonstrates that magnetic abrasive finishing (MAF) process using unbonded magnetic abrasives (UMA), can improve the quality of EDM machined surfaces effectively. The UMA used herein is a mechanical mixture of steel grit and SiC abrasive. SKD11 tool steel was used as the workpiece. Experimental results show that the recast layer and micro-cracks on EDM machined surfaces can be completely removed and a new surface of roughness on the order of 0.04 μm Ra can be produced. Additionally, experiments using the Taguchi method and L₁₈ orthogonal array enable the determination of the optimum process conditions for improving the surface finish. Further, the significance of the control factors was identified with the assistance of analysis of variance (ANOVA), and the optimum combination of the process parameters was verified by conducting several confirmatory experiments.     

Fretting fatigue crack initiation lifetime predictor tool: Using damage mechanics approach

Fretting fatigue is a combination of two complex mechanical phenomena. Fretting appears between components that are subjected to small relative oscillatory motions. Once these connected components undergo cyclic fatigue load, fretting fatigue occurs. In general, fretting fatigue failure process can be divided into two main portions, namely crack initiation and crack propagation. Fretting fatigue crack initiation characteristics are very difficult to detect because damages such as micro-cracks are always hidden between two contact surfaces.In this paper Continuum Damage Mechanics (CDM) approach in conjunction with Finite Element Analyses (FEA) is used to find a predictor tool for fretting fatigue crack initiation lifetime. For this purpose an uncoupled damage evolution law is developed to model fretting fatigue crack initiation lifetime at various fretting condition such as contact geometry, axial stress, normal load and tangential load. The predicted results are validated with published experimental data from literature.     

Si_3N_4陶瓷旋转超声磨削加工的表面摩擦特性

为了探索结构陶瓷材料在摩擦过程中表面形貌的变化规律及其对摩擦特性影响,分析了摩擦过程中材料的接触过程及力学关系,并对旋转超声磨削加工的Si3N4陶瓷试样开展了摩擦表面形貌、摩擦因数等特性的试验研究。首先根据接触特点和材料特性,基于分形理论推导出接触面总载荷计算公式,基于该公式建立了结构陶瓷摩擦因数分形模型。分析结果表明:当初始表面轮廓分形维数分别为1.4,1.45,1.5和1.55时,摩擦因数与摩擦后表面轮廓分形维数呈类似正态分布曲线。然后通过旋转超声磨削加工的Si3N4陶瓷试样面面接触摩擦试验,研究了摩擦后陶瓷材料表面微观形貌和摩擦因数变化规律,分析了各因素对摩擦因数的影响。试验结果表明:产生微观裂纹是Si3N4陶瓷摩擦后表面微观形貌的显著特点;温度值等于160℃是Si3N4陶瓷摩擦因数由下降转为上升的拐点;当施加载荷为360N和往复频率为80Hz时,摩擦因数最大。得到的结果为通过表面形貌控制提高结构陶瓷耐磨性能提供了技术支撑。     

改善CVD金刚石薄膜涂层刀具性能的工艺研究

用热丝CVD法,以丙酮和氢气为碳源,在WC-Co硬质合金衬底上沉积金刚石薄膜,在分析了工艺条件(衬底温度、碳源浓度、反应压力)对金刚石薄膜性能的影响的基础上,提出了分步沉积法改善金刚石薄膜涂层刀具性能的新工艺.结果表明,合理控制工艺条件的新工艺对涂层薄膜质量、形貌和粗糙度、薄膜与衬底间的附着力、刀具的耐用度及切削性能有显著影响,对获取实用化的在硬质合金刀具基体上沉积高附着强度、低粗糙度金刚石薄膜的新技术具有重要的意义.     

Thermoelastohydrodynamic Analysis of Spur Gears with Consideration of Surface Roughness

Thermoelastohydrodynamic lubrication (TEHL) analysis for spur gears with consideration of surface roughness is presented. The model is based on Johnson’s load sharing concept where a portion of load is carried by fluid film and the rest by asperities. The solution algorithm consists of two parts. In the first part, the scaling factors and film thickness with consideration of thermal effect are determined. Then, simplified energy equation is solved to predict the surfaces and film temperature. Once the film temperature is known, the viscosity of the lubricant and therefore friction coefficient are calculated. The predicted results for the friction coefficient based on this algorithm are in agreement with published experimental data as well as those of EHL simulations for rough line contact. First point of contact is the point where the asperities carry a large portion of load and the lubricant has the highest temperature and the lowest thickness. Also, according to experimental investigations, the largest amount of wear in spur gears happens in the first point of contact. Effect of speed on film temperature and friction coefficient has been studied. As speed increases, more heat is generated and therefore film temperature will rise. Film temperature rise will result in reduction of lubricant viscosity and consequently decrease in friction coefficient. Surface roughness effect on friction coefficient is also studied. An increase in surface roughness will increase the asperities interaction and therefore friction coefficient will rise.     

Analysis of wheel and rail adhesion under wet condition by using elastic–plastic microcontact model

This paper presents a numerical model to investigate the adhesion characteristics of the wheel/rail contact with consideration of surface roughness under wet conditions. The elastohydrodynamic lubrication theory is used to obtain the load carried by water, and the statistical elastic–plastic microcontact model presented by Zhao–Maietta–Chang is applied to calculate the load carried by asperities contact. Meanwhile, the thermal influencing reduction factor is used to consider the inlet heating effects on the film thickness, and the change of water viscosity is also taken into consideration due to the flash temperature generated by the moving rough surfaces. Furthermore, the present work investigates the dependence of the wheel/rail adhesion coefficient on train speed, surface roughness amplitude, the initial temperature, the plasticity index and the maximum contact pressure under wet condition. Copyright © 2014 John Wiley & Sons, Ltd.     

Study of plowing and friction at the surfaces of plastic deformed metals

Experimental and analytical investigations of plowing and friction were conducted at the surfaces of well-polished lead, aluminum, copper, nickel, molybdenum, and tungsten to study the mechanism of the load/penetration dependency. The experimental tests were performed with a Nano-Indenter XP of MTS and a Scanning Probe Microscope (SPM), Nanoscope IIIa of Digital Instruments. In addition to make indentation and measure the hardness and Young’s modulus, the indenter was used to make scratches at the surface of metals under different normal load while the penetration depth and frictional force encountered during the scratching were recorded. The SPM, operated mostly in the contact mode, was used to examine the scratch profile. Under the test conditions, plastic deformation dominated at the surfaces of the metals. An analytical model was established to express plastically deformed contacts, based on plowing of a conical-shaped indenter with a hemispherical tip at a plastic deformed surface. Penetration depth and scratched volume were calculated, which is in good agreement with experimental observation. The frictional coefficient μ was also calculated with the model, which accounted for plowing as well as the adhesion force between the indenter and surface. Beside fair agreement of experimental data and calculated values on μ under the loads applied, the model indicated a dramatic rise in friction coefficient under very low loads, which was not observed in the tests. The discrepancy was discussed, and it was believed that the dramatic increase in μ is for the calculated μ and may be due to the assumed dominant contribution of adhesion force in actual contact load with decreasing external load, and it appears only the adhesion energy Δγ is significant. The actual adhesion energy Δγ between our diamond indenter and metal surface in our test condition might be smaller than the value used in calculation.     

Relationship Between the Friction and Microscopic Contact Behavior of a Medical Compression Stocking at Different Strains

The contact and friction behavior of a medical compression stocking (MCS) under different strains was investigated in friction and compression experiments against a mechanical skin model. In addition, the 3D topography of the MCS surfaces was analyzed in order to study the relationship between macroscopic friction and microscopic surface properties. The load dependence of friction coefficients was found to be in accordance with the adhesion friction model. The surface structure of MCS samples was considerably changed when varying the strain state, while friction coefficients remained comparable, indicating real contact areas independent of strain on the microscopic level. The experimental findings could be confirmed and explained on the basis of the microscopic surface analyses, when interpreting the fabric surfaces to be composed of numerous individual round asperities obeying the Hertz contact model.