Tribological behaviors in microforming considering microscopically trapped lubricant at contact interface

In the study of microforming meeting the needs of miniaturization of parts to be formed, the size effects are important parameters to be considered seriously. The objective of the investigation is to establish an explicit friction model in micro/mesoscale to calculate the coefficient of friction (COF) considering size effects, which is very helpful in analysis of microforming processes. With the open–closed pocket assumption, a scaling factor was adopted to describe the size effects on tribological behaviors in microforming. Based on the general Wanheim/Bay friction law, a relationship between the real contact area and the forming load was obtained considering the microscopical contact interface and the pressure induced by the trapped lubricant liquid. An explicit equation was developed including fraction of real contact area, scaling factor, and properties of lubricant. The effects of scaling factor and lubricant properties were discussed by analyzing its effects on the fraction of real contact area and coefficient of friction. With the developed model, the coefficient of friction was calculated and introduced into the finite element simulation of micro-upsetting deformation using ABAQUS software. When the scaling factor is less than 9, the size effect of friction becomes the main reason which affects the shape parameter in micro-upsetting deformation. Comparisons show that simulation results are in good agreement with that of experiments, which means that the developed model is suitable for analyzing size effects of friction in microforming.     

Characterisation of the flattening behaviour of modelled asperities

In metal-forming processes tribological conditions between tool and workpiece are of greatest importance for process quality and process feasibility. This is even truer for microforming applications, where at least two dimensions of the workpiece are in the sub-millimetre range, due to increasing friction when process dimensions are scaled down. This effect can be explained by the model of open and closed lubricant pockets characterising the workpiece surface and the invariance of topography to scaling. As the number of workpiece asperities contacting the tool is drastically reduced the flattening behaviour of single asperities is of major interest for characterising tribology in microforming processes in more detail. Especially, a topography emerging on top of flattened asperites, the so-called nanotopography and its impact on the friction conditions has to be considered. Modelled asperities represented by pyramids with a base area of 120 μm × 120 μm and a height of 32 μm are flattened with a high-resolution experimental setup which enables in situ observation of the contact area. In-process measurement is complemented by post-process analysing the topography by confocal microscopy and scanning probe microscopy. This paper will show that a nanotopography on top of flattened asperities can emerge under certain geometrical conditions and that it has an impact on the friction conditions in the tool/workpiece interface. The detailed knowledge about the evolution of surface topography is relevant in particular to microforming but also for an improved understanding of tribological phenomena in general.     

Experimental and analytical studies of dynamic friction at the head/disk interface

Friction is an important parameter that critically impacts the tribological performance of a head/disk interface. The head/disk interface with laser zone texture affords a model system for the study of dynamic friction by virtue of its precisely-controlled contact geometry. By using two types of head sliders, i.e. the conventional slider and the padded slider, and a matrix of hard disks with a wide range of laser zone texture parameters, head/disk contacts involving a small number as well as a large number of bumps are realized. A rich variety of dynamic friction behaviors are observed with respect to bump height and bump density variations. To shed new light on the nature of HDI dynamic friction, an analytical model that treats both the deformational and the adhesive friction components on equal footings is formulated. It is shown that, based on the model analysis, the friction is deformation-dominated for HDIs involving a small number of contacting bumps and adhesion-dominated for HDIs involving a large number of contacting bumps. In the former case the friction decreases with bump density, whereas in the latter the friction increases with bump density.     

微成形技术的研究现状

阐述了微成形技术的基本概念，分析总结了微成形的研究现状、主要研究领域。以微成形试件微小化后尺寸效应的本质特征为基础，讨论了材料成形过程中的摩擦、材料流动规律的尺寸效应、试件的精度和成形设备的选择等问题，并进一步指出了微成形技术的发展方向。     

考虑表面粗糙度的湿式离合器啮合过程分析

湿式离合器在实际啮合过程中,离合器摩擦片高速运转,摩擦副接触为2个随机粗糙表面的接触,因此分析离合器啮合过程必须考虑摩擦片惯性及摩擦副表面粗糙度的影响.采用Greenwood-Tripp两粗糙表面接触模型,并考虑惯性的影响,对主动盘下表面有摩擦材料的湿式离合器的啮合过程进行分析,建立基于Patir-Cheng平均流量模型的研究模型,推导出摩擦副的润滑控制方程,并进行求解.理论计算得出的啮合转矩变化与实验结果相吻合,验证了理论分析的正确性.     

塑性微成形技术的发展

阐述了塑性微成形研究的目的和意义，重点从尺寸效应、本构模型、摩擦规律、冲压性能和超塑性微成形等5个方面描述了国外塑性微成形技术的发展现状，简单概括了国内微成形技术的发展状况，指出了这一技术面临的挑战和发展趋势。     

A review on the state-of-the-art microforming technologies

Product miniaturization is an emerging trend for facilitating product usage, enabling unique product functions to be implemented in micro-scaled geometries and features, and further reducing product weight and volume. Recently, a demand for microparts increased significantly in many industry clusters. Development of the advanced micromanufacturing technologies for fabrication of such microparts has thus become a critical issue. Microforming, which offers attractive characteristics including high productivity, low cost and good quality of the formed parts, provides a promising approach to fabricating metallic microparts. In the last two decades, a lot of effort has been made to the researches on size effect related deformation behaviors in microforming process and the development of the process. Having a panorama of these researches is necessary to support micropart design and development via microforming, and further advance this micromanufacturing process. In this paper, an intensive review on the latest development of microforming technologies is presented. First of all, the paper is focused on the review of the size effect-affected deformation behaviors and the mechanisms of the changes of flow stress, flow behavior, fracture behavior, elastic recovery, tooling–workpiece interfacial friction and the surface finish of the formed parts. The state-of-the-art microforming processes, including micro deep drawing, microembossing, micropunching, microcoining, microextrusion, microheading, and micro progressive forming are then presented. Finally, some research issues from the implementation of mass production perspective are also discussed.     

A Study on the Tribological Behavior of Surface Texturing on Babbitt Alloy under Mixed or Starved Lubrication

Aiming at improving the tribological performances of sliding bearings in mixed or starved lubrication regime, textures in square and linear radiating arrays are ablated on the surface of Babbitt alloy disks by laser radiation. Series of pin-on-disk wear tests and computational fluid dynamics (CFD) analysis are conducted to investigate the influence of distribution and geometry of the textures under various operating conditions. Results suggest that surface texturing suitably interacts with the material properties of Babbitt metal favorably improving its tribological performance. Friction coefficients of the disks with textures arrayed in square are generally much lower and more stable as compared to their counterparts with textures arrayed in linear radiation. Also, textures arrayed in a square with an area density of 8.6 % allow the lowest friction coefficient, as low as 0.015, to be achievable. Theoretical analysis sheds the light that proper texture arrangements tend to generate favorable distribution of micro-hydrodynamic pressure to improve the tribological performance of Babbitt alloy significantly.     

H62黄铜微成形镦粗实验研究

微成形是微细加工技术（microfabrication technology）群体中的一项技术。研究表明：零件微型化导致了尺寸效应，使传统的塑性加工工艺不能直接应用于微成形，尺寸效应阻碍了微成形的产业化。现通过对H62黄铜的微成形镦粗实验初步验证了由于尺度效应的影响，随着样件尺寸的减小，流动应力也呈现减少的趋势。     

Comparative tribological behaviors of TiN, CrN and MoNCu nanocomposite coatings

The purpose of this study is to investigate comparative tribological behaviors of Cu-doped TiN, CrN, and MoN coatings under a wide range of dry sliding conditions. TiN and CrN coatings have been developed and used by industry in numerous tribological applications including, machining, manufacturing and transportation. In contrast, MoN has attracted very little attention as a tribological coating in the past, despite being much harder than both TiN and CrN. In this paper, we will mainly concentrate on the Cu-doped versions of these coatings whose tribological properties have not yet been fully explored. The results of this study have confirmed that the addition of Cu into TiN, CrN and MoN coatings has indeed modified the grain size and morphology, but had a beneficial effect only on the friction and wear behavior of MoN. The tribological behavior of CrN did not change much with the addition of Cu but that of TiN became worse after Cu additions. Raman spectroscopy technique was used to elucidate the structural and chemical natures of the oxide films forming on sliding surfaces of Cu-doped TiN, CrN and MoN films. The differences in the friction and wear behavior of Cu-doped TiN, CrN, and MoN is fully considered and a mechanistic explanation has been provided using the principles of a crystal chemical model that can relate the lubricity of complex oxides to their ionic potentials.     

3维复杂成形数值模拟边界界面约束处理技术研究

边界界面动态接触约束与接触判断是3维复杂体积成形有限元数值模拟的关键技术问题之一。探索了3维复杂成形数值模拟接触状态搜索、接触摩擦以及界面滑动约束处理技术，提出基于模具型腔B样条曲面描述的接触状态区域层次搜索算法，以及引入边界摩擦与滑动约束的矩阵变换法，并进行了数值实验与齿轮镦锻成形有限元分析。     

Matrix-presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting

The main purpose of this paper is to develop the matrix presentation linear least square error method of inverse elastic-plastic large deformation finite element model for upsetting to obtain the friction coefficients during the upsetting process. This inverse model assumed the linear material and based on the modified experimental loading increments using the linear modified experimental upsetting loading standard proposed in this paper. Then the friction coefficients of contact boundary between the workpiece and the die at specific finite element analysis stages can be derived. Finally, using the cubic spline fitting, the history of friction coefficient during the upsetting process can be obtained. It is demonstrated that the workpiece profile of upsetting experiment is quite identical to the workpiece profile of simulation using the result obtained in this paper as the history of friction coefficient of contact boundary, and furthermore the distribution of stress and strain of the workpiece during upsetting process can be understood.     

摩擦学尺寸效应及相关问题的思考

根据文献研究,简单阐述了尺寸效应的研究现状、宏观/微观摩擦学的差异。从尺寸大小的角度,提出了应当对摩擦学传统理论有效性、过渡状态摩擦学、摩擦系数、摩擦学与材料、系统结构和物理规律尺寸效应与摩擦学特性的关系等一些摩擦学领域的重要基础问题进行认真思考和系统研究。     

Geometrical Aspects of the Tribological Properties of Graphite Fiber Reinforced Polyimide Composites

A Latin-square statistical experimental test design was used to evaluate the effect of temperature, load, and sliding speed on the tribological properties of graphite fiber reinforced polyimide (GFRPI) composite specimens. Hemispherically tipped composite riders were slid against 440C HT stainless steel disks. Comparisons were made to previous studies in which hemispherically tipped 400C HT stainless steel riders were slid against GFRPI composite disks and to studies in which GFRPI was used as a liner in plain spherical bearings. The results indicate that sliding surface geometry is especially important, in that different geometrics can give completely different friction and wear results. Load, temperature, and sliding distance were found to influence the friction and wear results but sliding speed was found to have little effect. Experiments on GFRPI riders with 10 weight percent additions of graphite fluoride showed that this addition had no effect on friction and wear.     

Prediction of residual stress distribution after turning in turbine disks

The state of a surface region after machining is definitely affected by cutting parameters, such as cutting speed, feed rate, tool nose radius, tool rake angle and the presence of a cutting fluid, which plays a major role in determining friction at the tool–chip interface. The aim of the present study is to develop a finite element model based on the general-purpose nonlinear finite element code MSC.Marc by MSC.Software Corporation. This software is capable of simulating the cutting process of low-pressure turbine disks of aircraft jet engines from its very beginning to steady-state conditions. Basically, the present analysis is a coupled thermo-mechanical dynamic-transient problem, based on the update Lagrangian formulation; no pre-defined path is given for the separation of the chip from the workpiece, since material deformation occurs as a continuous indentation performed by the rigid tool. In addition to the cutting parameters, the main inputs in this analysis are material constitutive data, the friction coefficient at the toolchip interface and the cutting tool temperature. All the relevant variables, like stresses, strains, temperatures, chip shape and residual stresses, are predicted in a wide range of cutting conditions. The results from the model are compared to some basic theories of metal cutting and to an experimental study, concerning orthogonal cutting of steel AISI 316L. Concerning the specific case of turning process of nickel alloy Inconel 718 low-pressure turbine disks, the calculated residual stress are compared to experimental measurements from real machined disks.     

Tribology of Skin: Review and Analysis of Experimental Results for the Friction Coefficient of Human Skin

In this review, we discuss the current knowledge on the tribology of human skin and present an analysis of the available experimental results for skin friction coefficients. Starting with an overview on the factors influencing the friction behaviour of skin, we discuss the up-to-date existing experimental data and compare the results for different anatomical skin areas and friction measurement techniques. For this purpose, we also estimated and analysed skin contact pressures applied during the various friction measurements. The detailed analyses show that substantial variations are a characteristic feature of friction coefficients measured for skin and that differences in skin hydration are the main cause thereof, followed by the influences of surface and material properties of the contacting materials. When the friction coefficients of skin are plotted as a function of the contact pressure, the majority of the literature data scatter over a wide range that can be explained by the adhesion friction model. The case of dry skin is reflected by relatively low and pressure-independent friction coefficients (greater than 0.2 and typically around 0.5), comparable to the dry friction of solids with rough surfaces. In contrast, the case of moist or wet skin is characterised by significantly higher (typically >1) friction coefficients that increase strongly with decreasing contact pressure and are essentially determined by the mechanical shear properties of wet skin. In several studies, effects of skin deformation mechanisms contributing to the total friction are evident from friction coefficients increasing with contact pressure. However, the corresponding friction coefficients still lie within the range delimited by the adhesion friction model. Further research effort towards the analysis of the microscopic contact area and mechanical properties of the upper skin layers is needed to improve our so far limited understanding of the complex tribological behaviour of human skin.     

Friction and wear effects on a micro/nano‐scale

In this paper are described tribological effects which can be found in micro‐tribological systems, and in those macro‐systems which can be analysed by micro‐methods, e.g., by atomic force microscopy (AFM) or related methods. Micro‐tribology systems have friction contacts with loads in the micro/nano‐newton range and/or dimensions in the micro/nanometre range. Experiments on the micro/nano‐scale should be easier to explain by theoretical modelling due to their simpler system structure. An example is discussed of adhesion and friction measurements between AFM tips and clean, flat, solid surfaces in ultra‐high vacuum, which shows some of the special aspects of micro/nano‐tribology. Surprising friction characteristics on surfaces with an artificial micro‐structure can be explained by skilled and careful topographical analysis of the friction path with an AFM. In micro‐sensor contacts, ‘single wear events’ can be detected using AFM analysis of the contact region. For ceramic compounds, different friction levels for the components of the material can be found. The problems, difficulties, and dangers of misinterpretation are also discussed.     

Effect of the Synergetic Action on Tribological Characteristics of Ni-Based Composites Containing Multiple-Lubricants

Ni-based self-lubricating composites with multiple-lubricants addition were prepared by a powder metallurgy technique, and the effect of multiple-lubricants on tribological properties was investigated from room temperature to 700?°C. The synergetic effects of graphite, MoS₂, and metallic silver lubricants on the tribological characteristics of composites were analyzed. XRD analysis showed that new Cr _x S _y and Mo₂C phase were formed in the composites containing graphite, MoS₂ and metallic Ag lubricants during the sintering process. The average friction coefficients (0.69?C0.22) and wear rates (11.90?C0.09?×?10^?5?mm³?N^?1?m^?1) were obtained when rubbing against Inconel 718 alloy from room temperature to 700?°C due to synergetic lubricating action of multiple-lubricants. A smooth lubricating was gradually generated on the worn surface, and the improving of tribological properties was attributed to the formation of lubricious glaze film on the worn surface and their partially transferred to the counterface. The graphite played the main role of lubrication at room temperature, while molybdate phase and graphite were responsible for low friction coefficients and wear rates at mid/high temperatures. The synergetic lubricating effect of molybdate (produced in the rubbing process at high temperatures) iron oxide (transfer from disk material to the pin) and remaining graphite multiple-lubricants play an important lubricating role during friction tests at a wide temperature range.     

Analysis of size effect on flow-induced defect in micro-scaled forming process

In microforming process, flow-induced defects caused by the irrational material flow in forming process have a significant effect on the quality of micro-formed part. In design of micropart and microforming process, this type of defects needs to be analyzed and the formation mechanism identified, such that the defects can be predicted and avoided via the rational design of micro-formed parts and microforming process. To address this issue, the size effect affected flow, and deformation behaviors need to be investigated. To explore how the size effects affect the flow-induced defects, experimental studies on the influence of a few significant parameters including geometry and grain sizes on the degree of flow-induced defects in microforming process of pure copper were conducted in this research. The flow-induced defects in microforming of a designed part are investigated, and the microstructure and flow pattern in micro-scaled extrusion of the parts with complicated shape are also studied. Based on the experimental results, the formation of folding defects is mainly affected by geometry size. The folding defect-free deformation occurs in the cases with coarse grains such that the parts with coarse grains do not have flow-induced defects, but some grains are broken and become potential insecurity factor. Furthermore, the relationship between grain and geometry sizes under which the flow-induced defects and grain broken can be successfully and simultaneously avoided is identified.