Fast Fourier Transform Analysis of Computer-Acquired Friction Data©

A commercial computer program for data acquisition and analysis has been used to facilitate the evaluation of the frequency content of friction force data obtained from a sliding wear test. The program uses a fast Fourier transform (FFT) algorithm to calculate the frequency spectrum of the friction force. The validity of the FFT program has been demonstrated by applying it to a recently published example that used a detailed mathematical treatment to remove an oscillatory component from friction data. When applied to friction force data from a pin-on-disk sliding wear test of NBD 200 silicon nitride sliding on NBD 100 silicon nitride, filtering of the dominant frequencies revealed by the FFT method reduces both the general dispersion of the friction values and almost completely eliminates localized signal perturbations. Successive filtering steps permit the association of particular frequencies with observed signal disturbances. Although the mean friction value in this example is not greatly altered by the frequency filtering, knowledge of the existing vibration frequency components in the tribosystem is important for a complete interpretation of wear mechanisms and for characterization of the machine/specimen interaction.     

Techniques in the study of impact and sliding wear of Zircaloy-4

A wear test apparatus has been designed to conduct detailed measurements of the contact forces and motion during impact and sliding (and combinations of the two). After carrying out pure impact and sliding tests on Zircaloy-4 couples in air at room temperature, it appears that the test apparatus has acceptable dynamic characteristics for wear studies.In pure impact, the wear rate of Zircaloy-4 was primarily influenced by the initial impact force which was measured dynamically. Secondary vibrations had a minimal effect on the wear rate.The wear rate in pure sliding was dependent upon the characteristics of the apparatus, such as its compliance, and the test conditions. Consequently the sliding amplitude should be monitored continuously throughout the test. In general, sliding wear was more severe than impact wear and good correlation between the average sliding amplitude and weight loss was observed. An indication of the relative motion between the wear specimens was given by the magnitude and form of the coefficient of friction which tended to increase with weight loss.     

Sliding wear of polymeric composites

The wear mechanisms of two different polymeric composites sliding against metal were investigated experimentally. The sliding distance, normal force and sliding speed were the test variables.Microscope observations showed that the mode of wear of the two materials is similar. Subsurface deformation, crack nucleation at the matrixharder particle or matrix-glass fibre interface, crack propagation parallel to the surface at a depth corresponding to the friction coefficient and crack shearing to the surface were found in both materials. The wear particles often have the shape of thin sheets. Comparison of the metal subsurfaces and wear particles with those of the polymers indicates that the mechanism of wear of semicrystalline composites is similar to delamination wear. Since film transfer greatly influences friction and wear it should be considered together with crack growth in fatigue in the prediction of wear of polymeric composites.     

刚性接触网中浸金属碳滑板在不同法向载荷下的载流摩擦磨损性能

为了更好地模拟浸金属碳滑板的实际应用工况,设置试验滑动距离为1 000 km,使用环-块式载流摩擦磨损试验机模拟地铁列车在刚性接触网系统中的运行条件,研究浸金属碳滑板在不同法向载荷作用下的载流摩擦磨损性能。试验结果表明：随着法向载荷的增大,摩擦因数不断增大,电弧能量下降,载流效率不断升高,滑板的温度以及磨损率都呈现出下降的趋势;随着滑动距离的增加,滑板的磨损率逐渐降低最后趋于稳定。SEM电镜观察结果表明：法向载荷较低时,浸金属碳滑板表面产生了较多烧蚀坑和裂纹,其磨损形式主要表现为电弧烧蚀以及片状剥落;随着法向载荷增大,滑板表面出现划痕和磨屑,磨粒磨损现象较为明显。研究表明：适当增大法向载荷可以有效抑制电弧的烧蚀作用,减少滑板表面的裂纹和烧蚀坑,从而降低滑板的磨损。     

Investigation of Wear by Electron Diffraction

This review of electron diffraction theory and technique emphasizes its scope and limitations in the study of lubrication. The first section describes important areas of wear and friction studies where the electron diffraction method may be used, with examples chosen from the literature. Included in the discussion are: (a) monolayers, liquid lubricants, and greases, (b) polished and amorphous films, (c) the relation to wear of the crystallite size of surface phases, (d) possible relations between wear and the orientation of surface films, (e) solid lubricants, (f) mechanism of extreme pressure lubrication, and (g) the identification of wear debris.

The second part of the paper presents the electron diffraction results from three wear studies in the authors' laboratories.

Molybdenum isobutylxanthate is an effective extreme pressure oil additive for hard steel. Electron diffraction shows that the anti-wear mechanism may in part be due to the formation of a film containing iron monosulfide.

Cast iron top-piston-ring durability in a spark-ignition engine was studied using several fuels which gave widely different wear rates. The composition of the surface layers of worn rings possibly does not depend on fuel additive, but is dependent on test length. The degree of orientation of occluded graphite which has been smeared over the surface is primarily a function of wear rate.

Finally, preliminary studies suggest that there may be a relation between ring-bore wear and the crystallite size of the solids of combustion of fuel additives.     

钢背衬碳纤维织物/环氧复合材料干摩擦特性研究

研究了钢背衬碳纤维织物/环氧复合材料在环-环端面干摩擦状态下的摩擦学特性,考察了MoS2与石墨粉及其配比、衬层厚度、法向载荷对衬层干摩擦性能的影响,用扫描电子显微镜对衬层的磨损表面及对偶件45^#钢环表面进行了观察与分析。结果表明：厚度为1.5mm的试环衬层在摩擦过程中主要表现出粘结磨损特性,而含20%（质量分数）MoS2粉的0.6mm衬层表现出疲劳磨损与磨粒磨损特性。摩擦因数-时间特性曲线表明MoS2粉在降低衬层摩擦因数的同时能够抑制环氧树脂向对偶钢环表面的粘结;石墨对衬层的减摩效果优于MoS2粉,但摩擦温升引起树脂向偶件表面转移增多使得减摩效果大大降低;质量分数为33%的MoS2与石墨粉衬层表现出最佳的摩擦学性能,衬层摩擦因数具有随载荷先减小后上升的趋势。     

Friction and wear behaviour of stainless steel rubbing against copper-impregnated metallized carbon

A series of experimental tests were carried out using stainless steel rubbing against copper-impregnated metallized carbon under electrical current on a pin-on-disc test rig. The test parameters include the sliding speed of 60-100 km/h, normal force of 40-80 N and electrical current of 0-50 A. During testing, the friction coefficient and wear volume were recorded. The topography of worn surfaces was also observed with SEM. The cross sectional profiles of worn surfaces of stainless steel were measured with Ambios profiler. The result displays that electrical current, normal load and sliding speed have a distinct effect on the friction and wear behaviour of stainless steel rubbing against copper-impregnated metallized carbon. Without electric current, the friction coefficient is largest but the wear volume of copper-impregnated metallized carbon is lowest. With increasing electric current, the friction coefficient decreases while the wear volume of copper-impregnated metallized carbon increases. Through the whole test, it is found that the wear loss of stainless steel was light. The wear of copper-impregnated metallized carbon becomes severe when electrical current or sliding speed is high. When the electrical current or sliding speed is high, arc ablation is a dominant wear mechanism of copper-impregnated metallized carbon.     

Tribological properties of short glass fiber reinforced polyamide 12 sliding on medium carbon steel

The friction and wear of short glass fiber reinforced polyamide 12 (PA12) were investigated. The behavior of the fibers on a sliding surface and their effect on the friction and wear were studied in terms of the amount and orientation of the fibers in the composite. Results showed that the friction level and wear resistance were strongly affected by the fiber content, and glass fiber patches produced on the sliding surface played important roles in the wear resistance of the composite. The optimum fiber content for the best wear resistance of the PA12 composite was approximately 30 wt.% and higher fiber contents had no added effect on the wear amount. The applied load also strongly affected the wear resistance due to the increase in temperature at the sliding interface, and an increase in rapid wear was observed when the interface temperature increased above the glass transition temperature of PA12. On the other hand, the fiber orientation had less effect on the friction and wear of the composite compared to the fiber content and applied load. Based on the behavior of glass fibers on the sliding surface and wear debris analysis, the wear mechanism of the PA12 composite is discussed.     

High speed tribological properties of graphite fiber/ Cu-Sn matrix composites

High speed dry friction experiments of graphite fiber/Cu-Sn matrix composites against steel were conducted at sliding velocities up to 235 m s^?1. The composite samples were prepared by the method of liquid metal infiltration. It has been determined that the friction coefficient and the wear rate depend on the amount of tin in the matrix, orientation of fibers relative to the sliding surface, the sliding velocity, the graphite grain size and the degree of liquid metal infiltration within the fibers. The increase in tin content leads to a decrease in both friction and wear due to an increase in matrix hardness. Specimens tested with the fibers oriented perpendicular to the sliding surface exhibit better frictional behavior than those with fibers parallel to the sliding surface. Both friction coefficient and wear rate reach a minimum value at a velocity between about 120 and 180 m s^?1. Large graphite grain size and poor liquid metal infiltration within the fibers have a detrimental effect on wear.     

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.     

Tribological roles of nanometre thick Ag layers on Hertzian macroscopic contacts

Abstract

Tribological performance of subnano to nanometre thick Ag layers deposited on Si(111) has been examined under ultra high vacuum conditions to understand effect of surface thin layers on the wear and friction characteristics. The slider was made of a diamond sphere 3 mm in radius. As a result, a minimum of the coefficient of friction 0·007 was observed over a film thickness range of 1·5–10 nm. The sliding planes were observed by Auger electron spectroscopy, reflection high energy electron diffraction (RHEED), synchrotron orbital radiated X-ray diffraction (SOR-XD) and scanning tunnelling microscopy (STM). No worn particles were found after 100 reciprocal sliding cycles, and the very low friction coefficient lasted for at least 1000 sliding cycles. Observations using STM on the sliding surfaces confirmed that the stacking Ag(111) planes slid. The SOR-XD and RHEED verified that a tribo-induced orientation of polycrystal film occurs as Ag(111) sliding planes are oriented parallel to the sliding direction on the track. The friction force of as deposited epitaxial Ag films as a function of the load was constant. On the other hand, in the 5 nm thick Ag films annealed to form complete single crystals, the friction coefficient showed a strong load dependency. At a load of 250 mN or more, the annealed films showed a low and static friction coefficient. These results suggested that the shearing resistance of nanometre thick Ag layers exhibits a strong anisotropic performance within the thickness range of nanometres, along with an orientation of Ag during sliding. Experimental results of sliding tests were discussed on the contribution of surface atoms to the friction, an extraordinarily low wear rate of the Ag layers, and the relationship between the nanoscopic structure and macroscopic tribological performance.     

Load dependence and lifetime studies of self‐assembled monolayers

A comparative study of the microtribological properties of native oxide on Si (100), Si (100) coated with octadecyltrichlorosilane (OTS) and perfluorode‐cyltrichlorosilane (FDTS) self‐assembling monolayers (SAMs) is presented. The frictional properties between these samples and a bare silicon sphere were examined using a microtribometer. Microfriction was investigated as a function of the normal load and relative humidity. Also, the microfriction of OTS‐ and FDTS‐coated surfaces was studied as a function of sliding time and normal load to examine the lifetime of these monolayers. Confirming the results of earlier studies, in the microtribological regime OTS and FDTS significantly reduce the friction force in comparison to the bare, native oxide covered (hydrophilic) silicon surface. The friction vs. normal load curve of oxide‐covered surfaces as well as the SAMs can be described by contact mechanics. Lifetime measurements of the SAMs, examined as a function of the normal load and relative humidity, indicate that the OTS monolayers wear quickly in both dry and moist environments, while the lifetime of FDTS monolayers appears to increase in moist environments. Copyright © 2006 John Wiley & Sons, Ltd.     

Deformation and wear behaviour of the junction in quasi-scratch friction

The initial scratching of soft metals by relatively hard metallic asperities involves considerable plastic deformation and wear of the harder metal. Thus the penetration effect on friction is reduced successively as sliding proceeds, leading to the shearing type of friction. Such a transition state of sliding can be defined as a quasi-scratch friction process because ploughing precedes the steady sliding condition.The deformation and wear behaviour at a friction junction was investigated using model experiments between a mild steel conical rider and a flat copper surface. Changes in geometry of the rider and pile-up of the flat metal were examined metallographically and with a microscope. It was found that a stable value of the friction force is determined from the geometric shape of the junction attained after the completion of transient sliding and the effect of initial asperity shape on the friction force becomes insignificant.     

The surface failure of fluorapatite single crystals

Single crystals of various material are used for basic studies of friction and wear because of their simplicity. Single pass sliding enables one to sort out the effects influencing wear such as anisotropy and environment. A review of the literature is given for studies with metallic single crystals, diamond, magnesium oxide, and sapphire. Results are presented for studies on fluorapatite single crystals. Frictional anisotropy was found to be similar to that in MgO single crystals even though fluorapatite and MgO have different lattice structures. A new finding is the fact that repeated sliding at contact stress states above a threshold value causes catastrophic surface damage after the second pass even though very little damage appeared on the first pass. Most previous studies report the results of single passes. In practical sliding systems where repeated sliding is always occurring, wear of brittle materials must be dominated by the catastrophic mechanisms found on the second pass rather than by the events that occurred during the first pass.     

腐蚀介质条件下NiCrBSi/WC喷熔层的摩擦学性能研究

采用氧-乙炔火焰喷熔工艺,制备了碳化钨颗粒增强镍基合金喷熔层(NiCrBSi/WC),研究了它在腐蚀介质条件下的摩擦磨损行为与机理,并考察了载荷、滑动速度对其摩擦磨损性能的影响规律。研究结果表明:NiCrBSi/WC具有良好的耐腐蚀磨损性能,且当WC含量为20%时,腐蚀磨损率最低;WC含量超过20%后,由于喷熔层存在“腐蚀原电池”效应,其腐蚀磨损率增大。NiCrBSi/WC的腐蚀磨损率随载荷增加而变大,随速度增大而减小。载荷的增加使喷熔层的犁削磨损加剧,导致摩擦系数和磨损率增大;速度的增大造成摩擦界面温度上升,可生成摩擦转移膜,从而降低了喷熔层的磨损率。     

Effect of Water Swelling on the Tribological Properties of PMMA Spin-Cast Film and Brush in Aqueous Environment

Due to their light weight, low corrosion and good tribological properties, polymer films have been widely studied in dry condition as well as recently in aqueous environment. Though the presence of water can further reduce the friction, it promotes the wear rate of the polymer films. As a remedy to decrease the wear rate of polymer films under aqueous condition, in this study, we used PMMA brush which is chemically anchored to a substrate and compared its friction and wear properties with those of conventional PMMA spin-cast film. Ellipsometry, contact angle measurements and atomic force microscopy are used to study the surface properties, e.g., wear mechanisms and wear depths of PMMA films. Under different sliding speeds and applied loads, PMMA brush showed lower friction than PMMA spin-cast film in aqueous. Moreover, it was shown that the swelling of water molecules is a dominant factor in determining the wear durability of PMMA films in which PMMA brush showed better wear performance than PMMA spin-cast film.     

Macro- and nanotribological properties of organosilane monolayers prepared by a chemical vapor adsorption method on silicon substrates

Organosilane monolayers are novel ultrathin films used to control the physicochemical properties, such as friction and wear, of solid surfaces. In this study, the authors prepared alkylsilane and fluoroalkylsilane monolayers with a series of chain lengths by a chemical vapor adsorption method. The monolayers tribological properties were investigated by lateral force microscope (LFM) and friction tester. LFM nanoscale measurements of tribological properties showed that alkylsilane monolayer gave lower lateral force than the Si substrate surface. The lateral force decreased as the length of the alkyl chain increased. On the macroscale, friction test revealed that the organosilane monolayers gave lower dynamic friction coefficients than the Si substrate surface in air at room temperature. The longer the alkyl chain, the greater the wear resistance of the organosilane monolayers. Friction experiments using tetradecane as a lubricant showed better tribological properties than were obtained in air. Furthermore, microscopically line-patterned two-component organosilane monolayers were prepared and their macroscopic friction behavior was investigated. Even though the height difference between the two-components was less than 1 nm, friction force anisotropy between the parallel and perpendicular directions against the line pattern was observed.     

Direct deformation study of AFM probe tips modified by hydrophobic alkylsilane self-assembled monolayers

The in-use wear of atomic force microscopy (AFM) probe tips is crucial for the reliability of AFM measurements. Increase of tip size for several nanometers is difficult to monitor but it can already taint subsequent AFM data. We have developed a method to study the shape evolution of AFM probe tips in nanometer scale. This approach provides direct comparison of probe shape profiles, and thus can help in evaluation of the level of tip damage and quality of acquired AFM data. Consequently, the shape degradation of probes modified by hydrophobic alkylsilane self-assembled monolayers (SAMs) was studied. The tip wear length and wear volume were adopted to quantitatively verify the effectiveness of hydrophobic coatings. When compared with their silicon counterparts, probes modified by SAM materials exhibit superior wear-resistant behavior in tapping mode scans.     

The Tribological Behavior of Polycrystalline Cobalt as Related to Crystallographic Texture and Structure

The friction and wear behavior of polycrystalline hexagonal cobalt was determined under normal atmospheric conditions, using heavy loads and sliding speeds ranging from 0.92 cm/sec to 36.8 cm/sec. Very uniform wear was observed for most conditions together with a low coefficient of friction. X-ray diffraction measurements showed that this desirable behavior is caused by the formation of a preferred orientation during sliding. Under high-load high-speed conditions a different mode of wear was observed, best described as brittle disintegration. When cobalt was alloyed with 8 percent iron and thus rendered face-centered cubic, a deeply torn wear track, an increased initial wear rate and a high coefficient of friction were obtained.     

Explaining the mechanics of metallic sliding friction and wear in terms of slipline field models of asperity deformation

It is shown that the force which opposes the sliding of a hard relatively smooth surface over a softer surface can be explained as the force needed to push waves of plastically deformed material along the soft surface ahead of asperities on the hard surface. For rougher surfaces and/or poorer lubrication it is shown how the wave can be torn off or material removed by a chip formation process and wear particles formed. Coefficients of friction predicted from the corresponding asperity deformation models are shown to give good agreement with experimental results. For smooth well lubricated surfaces the wear of the softer surface is shown to occur as a result of the progressive damage to this surface brought about by the repeated passage of waves across it. Equations for predicting wear are derived from the asperity deformation models and a comparison made between predicted and experimental wear results. The paper ends by considering possible future trends in research into the mechanics of friction and wear.