三维表面形貌的逾渗特性表征

为了建立表面形貌微观结构与其功能特性之间的关系,基于逾渗理论建立了表面形貌的逾渗模型,用逾渗概率、空体集团平均大小和空体集团分布系数对三维表面形貌的逾渗特性进行了量化表征。采用数字滤波技术生成具有给定自相关函数和纹理取向的数字化粗糙表面,分析了具有相同均方根粗糙度而结构不同的三维表面形貌的逾渗特性,给出了表面纹理方向参数和自相关长度对表面逾渗特性的影响,并借助部分三维形貌参数(ISO25178)建立了表面形貌与逾渗特性参数间的量化关系。结果表明:对于各向异性表面,沿横向搜索跨越空体集团,表面逾渗发生时的表面高度、逾渗阈值和逾渗体积均随着表面纹理方向参数的增大呈减小趋势,而空体集团分布系数呈增大趋势;沿纵向搜索时,其变化规律与横向相反。对于各向同性表面,逾渗发生时的表面高度和逾渗阈值随着表面自相关长度的增大呈先减小后小幅增大趋势,而逾渗体积和空体集团平均大小呈逐渐减小趋势。研究结果为面向功能的表面形貌设计提供了理论基础。     

激光表面织构对45钢干摩擦性能的影响

利用LM-YLP-20F-Ⅱ型激光打标机在45钢试件表面加工出具有规则排列的圆形微坑阵列。通过环-块线接触摩擦磨损试验,研究了激光表面织构对不同硬度45钢试件干摩擦磨损性能的影响。考察了激光微坑内部和外部的硬度,利用扫描电镜(SEM)观察了试件的磨损形貌,探讨了激光表面织构对不同硬度45钢试件干摩擦性能的影响机理。研究表明:激光表面织构的几何形貌对摩擦副的干摩擦磨损性能影响显著;在一定条件下,激光表面织构可以改善干摩擦的磨损性能;激光织构化处理会显著改变试件的表面硬度;当试件基体硬度较低时,微坑会支撑保护基体较软的材料,降低粘着磨损;而当基体硬度较高时,高硬度微坑对对偶面的磨粒磨削会增加,从而加剧摩擦副的磨粒磨损。     

表面微凹坑和纹理方向对界面摩擦的耦合影响

选取三种不同纹理的铝合金试样,并在试样上加工不同面积占有率的规则圆形微凹坑,利用自制的摩擦试验装置,在油润滑条件下以不同接触压力进行摩擦试验,试验过程中滑动方向与表面纹理方向的夹角分别为0°、45°、90°。利用非接触式三维轮廓仪测量试验前后试样的三维表面形貌,并选取Sa、Str、Vvv、Vvc等表面表征参数来分析滑动接触界面表面形貌的变化。结果表明：表面纹理方向的差异导致铝合金表面在滑动接触摩擦过程中表现出各向异性,而在其表面加工不同面积占有率的微凹坑,减弱了铝合金表面纹理方向性对界面摩擦的影响,反映出表面微凹坑和纹理对界面摩擦的耦合作用。同时界面摩擦对试件的表面形貌也有明显的影响,Str、Vvv、Vvc在试验后发生了规律性的变化。     

具有微织构形貌的结合面法向接触刚度分形模型的研究

研究微织构结合面上的表面形貌参数对结合面法向接触刚度的影响。根据微织构表面的形貌特征,将微织构表面分为织构前表面和织构区域两部分,由分形接触理论计算出织构前表面上微凸体的基本参数,忽略织构区域底部未接触部分,将微凸体在接触载荷作用下的变形分为三个阶段:弹性阶段、弹塑性阶段、塑性阶段。由接触力学理论,首先建立织构前表面上单个微凸体的法向接触刚度模型。然后由微观到宏观,结合微织构表面的织构形貌特征,构造整个宏观微织构结合面的法向接触刚度计算模型,研究不同的表面形貌参数对于微织构界面上法向接触载荷、微凸体因载荷产生的变形以及法向接触刚度的影响。经过仿真分析之后,结果表明,当微织构结合面的法向接触载荷不断增大时,结合面的法向接触刚度总体呈单调上升趋势;并且随着织构密度的增加,结合面上由于织构形貌的存在,使得一部分微凸体未产生变形,从而减小结合面的法向接触刚度,并且当法向载荷增加时,这种效果会更加明显;在相同的接触载荷下,塑性指数越高,处于塑性变形状态的微凸体就越多,从而使具有微织构形貌的结合面的法向接触刚度变大。     

多尺度点磨削表面纹理功能及摩擦学特性分析

为研究多尺度表面的表面形貌对其使用功能和摩擦学特性的影响,设计加工5组具有不同多尺度特征的点磨削表面纹理试件。测量试件的表面形貌功能参数,通过功能参数对比分析,得出试件表面形貌对其功能参数的影响规律。在油润滑和干摩擦两种情况下对试件进行表面摩擦学特性实验,分析后得到了多尺度点磨削表面形貌对表面摩擦学特性的影响规律。结果表明:多尺度点磨削表面具有加工可控性;不同表面形貌的零件,其表面功能特性也不相同。可通过控制工艺参数来获得合理的零件表面形貌及纹理特征,以满足零件表面实际应用及功能要求。     

磨削加工表面纹理的特征分析

表面形貌与加工表面摩擦、磨损、润滑性能密切相关.采用平面磨削的方法在机械零件常用材料45钢试件表面制作一些有规则的纹理(如横纹理、30°斜纹理、45°斜纹理),采用表面形貌统计参数中的轮廓高度算术平均值R<,a>、微观不平度十点高度R<,a>、轮廓微观不平度的平均间距S<,m>对表面形貌进行评价,分析了表面形貌表征参数与磨削表面纹理的相关关系.结果表明磨削表面形貌不仅与加工过程中的工艺参数密切相关,而且其纹理特征在很大程度上影响了表面形貌参数.     

微细电火花线切割加工表面凸峰的高斯曲率

微细电火花线切割表面微观三维形貌与传统切削加工表面完全不同,属于各向同性表面,在对其进行三维粗糙度评定时,测量方向对混合参数的影响很大,测量方向选择不当,混合参数的计算精度很低,甚至出现计算结果与被测表面实际观测结果相反的情况。根据微细电火花线切割加工表面凸峰的特点,采用高斯曲率表征其表面凸峰的凹凸性,消除了测量方向对计算结果的影响。经试验验证,高斯曲率对微细电火花线切割加工表面凸峰的表征结果准确,计算精度高。     

机械结合面接触模型有限元分析

借助LI-3型表面形貌测量仪对磨削加工的铸铁试件进行了三维形貌测量,基于实测表面数据逆向建模得到三维有限元模型,并导入ANSYS软件中建立该粗糙表面的接触模型。通过对不同采样长度下的机械结合面接触模型进行有限元仿真分析,研究了其接触面积、最大变形量和接触压力随载荷的变化关系。结果表明,采样长度越小,越能体现表面形貌精细结构,微凸体分布更加均匀。相同粗糙度、不同采样长度下表面接触特性的探索对于从微观方向解释宏观现象,揭示结合面作用机理的本质,具有良好的参考价值。     

缸套表面形貌磨损特性的研究

在缸套磨损的不同阶段 ,其表面将呈现出不同的表面形貌特性 ,定量描述上述特性对表面的功能分析及表面形貌设计具有很大的帮助。本文通过对缸套表面的实测和分析 ,研究了在磨损前后表面形貌的变化 ,并首次定量描述了微凸体密度、微凸体曲率半径及其沿高度方向的变化     

螺栓结合面微观接触特性的不确定性量化

针对螺栓结合面微观接触特性具有不确定性，传统基于确定性理论建立的模型难以完整表征结合面微观接触特性的问题，提出了一种基于蒙特卡洛法的螺栓结合面微观接触特性的不确定性量化方法。首先，基于分形理论，表征了同一粗糙度结合面微凸体轮廓高度，并采用矩谱法求解了结合面表面形貌参数区间；其次，利用中心极限定理，将表面形貌参数区间变为符合微凸体轮廓高度分布的高斯分布函数，解决了随机抽样误差的累加造成的置信水平降低；最后，将表面形貌参数的不确定性嵌入蒙特卡洛法，获得了结合面接触特性的区间估计，通过对比分析揭示了考虑不确定性因素时接触间隙对接触特性的影响规律。研究表明，表面形貌参数的不确定性对螺栓结合面接触特性变化具有显著影响，并导致不确定性的影响不断累加。该方法为准确量化螺栓结合面的不确定性提供了理论依据与参考。     

Influence of Friction on Surface Asperity Flattening Process in Cold Uniaxial Planar Compression (CUPC)

Atomic force microscope and electron back-scattering diffraction measurement methods were used to study the effects of friction on surface asperity flattening and surface texture during the uniaxial planar compression of annealed aluminum alloy. With an increase in gauged reduction, surface asperity tended to be flattened. Friction could boost the surface asperity flattening process by reducing the flow stress in deformation. The development of surface asperity features demonstrated that friction can effectively hinder the development of the Goss orientation component {011} <100> and clearly promote the generation of brass orientation {011} <100> orientation. Regardless of whether the sample was compressed with lubricant or not, a few S orientation component {123} <634> formed in sample edge area.     

A Two-Wavelength Model of Surface Flattening in Cold Metal Rolling with Mixed Lubrication

A new model of surface flattening is developed for cold metal rolling in the mixed regime. Longitudinal surface roughness is modeled by two separate wavelengths. The new model follows the asperity crushing analysis of Sutcliffe (1999) for unlubricated rolling but additionally includes a hydrodynamic model to account for the effect of the lubricant. The effect of various parameters including speed, reduction in strip thickness, roughness wavelength and lubricant properties is examined. The results show similar behavior to previous models of mixed lubrication, with a speed parameter A_s having the most influence, and confirm the results for unlubricated rolling that the short wavelength components of the surface roughness persist more than the long wavelength components. The predicted changes in roughness are in good agreement with experiments.     

Investigation of the strain distribution with lubrication during the deep drawing process

A lubrication/friction model can be implemented in FEM codes to predict the contact area ratio, friction coefficient and strain distribution in lubricated deep drawing process. In the lubrication analysis, the surface roughness effect on lubrication flow is included by using Wilson and Marsault's average Reynolds equation that is appropriated for mixed lubrication with severe asperity contact. With regard to the asperity contact theory, the well-known flattening effect is considered. Friction is expressed in terms of variables such as lubricant film thickness, sheet roughness, lubricant viscosity, interface pressure, sliding speed, and strain rate. The proposed lubrication/friction model combined with a finite element code of deep drawing process to predict the contact area ratio, friction coefficient and strain distribution. Numerical results showed that the present analysis provides a good agreement with the measured strain distributions.     

A microwedge model of sliding contact in boundary/mixed lubrication

A refined friction model of sliding contact in boundary/mixed lubrication regime is developed. In addition to the well-known asperity flattening and roughening effects, significant deformation of asperities can be incited by the elastic microwedges on the tool surface. A model of asperity deformation, which includes the effects of smoothing, roughening, and microwedge, is proposed for processes where smooth tool and rough workpiece are used. A finite element formulation incorporating the microwedge effect in the Reynolds equation for lubricant flow is also derived. Numerical results showed that the inclusion of microwedge in the analysis provides a good agreement with the experimental measurements, especially with many interesting phenomena that can be neither explained nor predicted by the other friction models.     

Crystal Plasticity Finite Modelling of 3D Surface Asperity Flattening in Uniaxial Planar Compression

Rate-dependent crystal plasticity constitutive model has been employed into finite element software ABAQUS to simulate surface asperity flattening in uniaxial planar compression. Measured textures and surface roughness are introduced into the 3D surface roughness model. The calculated results show a good agreement with the experimental results. With an increase of reduction, the surface asperity flattening tends to increase, and Goss texture {011} 〈100〉 and brass component {110} 〈112〉 become stronger, whilst the cubic texture {001} 〈100〉 becomes weaker. If the reduction reaches 40%, Schmid in-grain shear band appears and the strain localisation starts. The evolution of surface feature (roughness) shows the obvious sensitivity on the orientation {111} of near-top surface.     

Molecular Dynamics Simulation of Lubricant Redistribution and Transfer at Near-Contact Head-Disk Interface

When the spacing between the slider and lubricant in a hard disk drive decreases to less than 5 nm, the effect of the intermolecular force between these two surfaces can no longer be ignored. This effect on the lubricant distribution at the near-contact head disk interface is investigated via molecular dynamics method. In this study, the lubricant is confined between a smooth disk surface and a rough slider surface represented as a partially cosinusoidal wave. The simulation results reveal that the intermolecular force-induced meniscus formation at the near-contact head disk interface is strongly sensitive to the slider-to-disk separation, lubricant film thickness and the asperity shape (or roughness) of the slider. The attractive van der Waals forces between the slider and lubricant become weaker with increasing slider-to-disk separation and asperity mid-height, but decreasing lubricant film thickness and asperity mid-width. The Hamaker theory application to van der Waals interactions is also introduced to verify the molecular dynamics simulation. It is found that the critical separation, below which the lubricant will lose its stability to form a meniscus, increases approximately linearly with the lubricant film thickness, for slider surfaces with or without roughness both in the molecular dynamics simulation and Hamaker theory application to van der Waals interactions. Moreover, it is observed that the critical separation between a smooth disk and rough slider surface will slightly decrease when the asperity mid-height increases. The same phenomenon is observed when the asperity mid-width reduces.     

活塞环-缸套二维润滑特性的研究

基于二维平均流量模型和微凸体接触模型，研究了活塞环的二维润滑特性，并考虑了活塞系统偏摆、润滑油粘度变化及表面粗糙度等因素的影响。通过计算获得了活塞环－缸套间油膜厚度的二维分布。结果表明，油膜厚度沿周向是不均匀的。本文还对活塞环开口位置及偏摆的影响做了定量的分析。     

Effect of shot peening on rolling contact fatigue and lubricant film thickness within mixed lubricated non-conformal rolling/sliding contacts

The effect of shot peening on rolling contact fatigue (RCF) and lubricant film thickness within non-conformal rolling/sliding contacts operated under mixed lubrication conditions was observed in this study. Rolling contact fatigue tests and film thickness measurements were carried out using specimens with modified surface topography by shot peening process using glass beads having diameter between 0.07 and 0.11 mm. It has been shown that the effect of shot peening on RCF has no positive effect even if shot peened surface of the roller exhibited somewhat higher hardness in contrast to the grounded surface. The reduction of RCF may be caused due to asperities interactions because after shot peening the surface roughness of the roller was increased. Film thickness measurements confirmed that the contact is realized actually only between asperity peaks of shot peened ball and smooth disc.Conversely, no negative effect on RCF was observed when the shot peened surface of the roller was polished. The polish of asperity peaks causes the creation of lands and micro-cavities, which may be employed as lubricant micro-reservoirs. From film thickness measurements it has been observed that lubricant emitted by shallow micro-cavities can provide the local increase in lubrication film thickness, which thereby reduces asperities interactions. Similar results were obtained for start-up conditions where the squeeze lubricant enlarges film thickness and reduces surface interactions.From the obtained results, it can be suggested that properly designed surface topography modification could help to increase the efficiency of lubrication films leading to the enhancement of contact fatigue life of non-conformal mixed lubricated rolling/sliding contacts.     

Influence of surface roughness features on mixed‐film lubrication

An optical technique (three‐dimensional spacer layer imaging) has been developed to map accurately lubricant film thickness in thin‐film elastohydrodynamic (EHD) contacts. This experimental technique has been used to study the influence of surface roughness features, asperity height, and slope on EHD film thickness and pressure. Single ridges transverse to the entrainment direction were used to represent asperities. It was found that the ridges with lower slopes generate films of greater minimum thickness. Below a certain entrainment speed, the minimum film thickness declined at a rate dependent on the ridge slope. At low speeds, the ridges with higher slopes entrapped a larger volume of lubricant ahead of the ridge and along the entrainment direction. For all speeds, the highest ridges entrapped the most lubricant. Both ridge slope and ridge height had a negligible effect on mean film thickness in the contact. Asperity pressure increased with higher ridge slope, but was not influenced by entrainment speed. An increase in pressure was found where lubricant is entrapped upstream of a ridge.