Nonlocal Frictional Effects at Indentation of Elastic Materials

Indentation of elastic materials is investigated numerically using the finite element method. Large deformation theory is relied upon for accuracy. The study focuses on nonlocal frictional effects on relevant indentation quantities in the microindentation regime. The indentation quantities investigated include both local and global ones. It is shown that nonlocal frictional effects are small when global quantities are at issue, as is the case when conventional (Coulomb) theory of friction is used, also when these features are introduced at the ridges of a Vickers indenter where stress gradients are substantial. These effects are, however, shown to be of importance for particular indenter geometries as far as local field variables are concerned.     

摩擦界面内杂质颗粒物的力学行为研究进展*

摩擦界面内夹杂的固体颗粒物,会影响摩擦副的摩擦、磨损性能,并可能导致摩擦副异常振动和温度升高,应用表面织构可改善摩擦界面的摩擦学特性,为摩擦副的减摩、抗磨提供了有效的途径。介绍摩擦界面夹杂有固体颗粒时的摩擦、磨损机制,概述颗粒-摩擦副的力学模型、颗粒被卷吸进入摩擦接触界面前后的力学行为,以及颗粒存在时表面织构的作用。展望颗粒-摩擦副系统的研究方向,包括非球形颗粒的力学行为研究,摩擦接触界面间颗粒的运移规律研究,摩擦体系各要素间的协同作用和制约机制研究,润滑脂作为润滑剂时固体颗粒的力学行为研究等。     

Effects of Interfacial Bonding on Friction and Wear at Silica/Silica Interfaces

Static friction between amorphous silica surfaces with a varying number of interfacial siloxane (Si–O–Si) bridges was studied using molecular dynamic simulations. Static friction was found to increase linearly with the applied normal pressure, which can be explained in the framework of Prandlt–Tomlinson’s model. Friction force was found to increase with concentration of siloxane bridges, but with a decreasing gradient, with the latter being due to interactions between neighboring siloxane bridges. In addition, we identified atomic-level wear mechanisms of silica. These mechanisms include both transfer of individual atoms accompanied by breaking interfacial siloxane bridges and transfer of atomic cluster initialized by rupturing of surface Si–O bonds. Our simulations showed that small clusters are continually formed and dissolved at the sliding interface, which plays an important role in wear at silica/silica interface.     

Frictional Characteristics of Quasicrystals at High Temperatures

AlCuFe quasicrystal coatings were deposited by unbalanced magnetron sputtering for study of their friction and wear properties at high temperatures. It has been shown that growth and annealing conditions can be controlled to produce icosahedral quasicrystal or the approximant cubic phase. The comparison of friction and wear properties between quasicrystal and an approximant with nearly the same stoichiometry permits assessment of the unique quasicrystalline structure for tribological applications. The goals of this study are to determine how crystal structure influences tribological properties and to study the general friction and wear behavior of quasicrystals. Tribological properties were evaluated using a pin-on-disk tribometer and crystal structure was characterized using an X-ray diffractometer. The tests specimens were 10 m thick films deposited on a 1 diameter steel disk and a 1/4 diameter steel ball. Data was collected over a range of temperatures from room to 600°C. Friction coefficients and wear rates of quasicrystals and approximants were nearly identical for room temperature tests. The wear process generated Al, Cu, Fe, and oxide debris on the side of the track, but overall wear was mild. The friction coefficient of the icosahedral phase was 25% lower than the cubic phase at 150 thru 450°C. Generally, only moderate differences in the friction and wear properties were observed between the icosahedral quasicrystal phase and the cubic phase.     

Synergistic Effects of MoDTC and ZDTP on Frictional Behaviour of Tribofilms at the Nanometer Scale

The layered structure and the rheological properties of anti-wear films, generated in a rolling/sliding contact from lubricants containing zinc dialkyldithiophosphate (ZDTP) and/or molybdenum dialkyldithiocarbamate (MoDTC) additives, have been studied by dynamic nanoindentation experiments coupled with a simple modelling of the stiffness measurements. Local nano-friction experiments were conducted with the same device in order to determine the evolution of the friction coefficient as a function of the applied pressure for the different lubricant formulations. For the MoDTC film, the applied pressure in the friction test remains low (<0.5 GPa) and the apparent friction coefficient is high ( > 0.4). For the tribofilms containing MoDTC together with ZDTP, which permits the applied pressure to increase up to a few GPa through some accommodation process, a very low friction domain appears (0.01 < < 0.05), located a few nanometers below the surface of the tribofilm. This low friction coefficient is attributed to the presence of MoS₂ planes sliding over each other in a favourable configuration obtained when the pressure is sufficiently high, which is made possible by the presence of ZDTP.     

Frictional properties of elastomers at increased sliding rates

The paper proposes a method for estimating temperatures on the friction surface of elastomers at increased sliding rates on the assumption that over a wide range of rates and temperatures the friction conditions are determined by their relaxation characteristics, provided the mechano-chemical processes do not exert any significant effect. This enables one to apply the principle of temperature-time (rates) reduction and to estimate, from the dependence of the friction coefficient on the sliding rates, determined over a wide temperature range, the temperature regime of friction over a wide range of rates.     

A Method to Calculate Frictional Effects in Oil-Lubricated Ball Bearings

Friction and heat generation in oil-lubricated ball bearings is mainly the result of sliding in the ball-raceway contacts and agitation of the lubricant in the free space between the balls, cage. and bearing rings. Endurance of a ball bearing is highly dependent on the thicknesses of the oil films which serve to separate the balls from the raceways in a well-lubricated bearing. The film thicknesses, in turn, are dependent upon the lubricant's viscous properties. These are functions of bearing temperatures and are determined by the balance between the rates of frictional heat generation and heat dissipation. Therefore, in the design of a ball bearing application such as the high speed rotor and low speed rotor support bearings in an aircraft gas turbine, it is important to be able to predict the bearing frictional heat generation rates with reasonable accuracy. This paper presents a method to perform the required calculations considering hearing loading and speeds, realistic lubricant rheological properties, and a relatively simple heat transfer system between the hearing rings, halls. And lubricant. The results of the analysis are shown to compare favorably with experimental data.     

电流对载流摩擦副材料损伤行为的影响

载流摩擦过程中材料的损伤行为是影响载流摩擦副寿命的关键。采用纯铜与QCr05配副,研究纯铜材料的载流磨损行为。结果表明,载流条件下磨损行为仍然包括无电条件下的主要磨损行为;电流的介入导致磨损行为发生变化,在载荷50 N、滑动速度10 m/s条件下,电流从0增加到50 A时表面温度增加5.6~7.2倍;且摩擦表面严重粗糙化,发生剧烈不均匀塑性变形,在载荷70 N、滑动速度20 m/s、电流60 A条件下,其变形层厚度约180 μm;电流还直接导致材料的损伤,主要包括熔融和喷溅。     

Frictional mutation

Attention focuses on frictional mutation, which has been observed in the wear of frictional components.     

The Effects of Load on the Frictional Properties of Molybdenum Disulfide

The kinetic coefficient of friction was determined for several grades of commercially available molybdenum disulfide powder. These powders were individually applied to separate steel-supporting substrates to form a thin lubricant film. Friction measurements were made at loads ranging from 0.1 to 10 kg and at a slow speed of sliding. The results of Ike investigation showed that the coefficient of friction decreased with increasing load, contrary to Amontons' Second Law. A theory was postulated to explain the observed variation in the friction coefficient with load, It was concluded that the variation in the friction coefficient could be explained solely on the basis of the macroelastic and/or macroelastic and plastic deformation characteristics of the supporting substrates.     

Effects of Plasma-Polymerized Coatings on the Frictional Behavior of Beryllium Surfaces

Eight different polymers were coated on beryllium surfaces by glow discharge polymerization in an inductively coupled tubular reactor. Using a pin-on-disk apparatus, friction tests were then conducted on the polymer-coated beryllium disks with Pyroceram pins. Of all the coated surfaces, polyethylene-coated beryllium exhibited the lowest friction with no measurable wear. The friction coefficient of the polyethylene-coated beryllium surface was found to depend strongly on the film thickness. The variation of friction coefficient with film thickness was analyzed, and a reasonable agreement between theoretical prediction and experimental data was obtained.     

Frictional effects on flow stress determination at high temperatures and strain rates

It has been reported recently that the average die pressure as measured during the initial compression of non-lubricated cylinders was less than 5% greater than that for lubricated samples, when the final diameter-to-height ratio is less than 2. These tests were carried out at room temperature and low strain-rates.To determine whether these results occur at high temperature and high strain-rates, cylinders of hot-rolled low-carbon steel were upset under lubricated and non-lubricated conditions. The steel samples were deformed by a cam plastometer at constant true strain-rates of 2.0, 20.0 and 140.0 s⁻¹. The results clearly show that the average die pressure for lubricated and non-lubricated samples are, within experimental accuracy, equal. Further testing demonstrated that the average die pressure does not depend on sample geometry for cylinders with initial aspect ratios of 0.5, 0.65 and 0.85, deformed to a true strain of 0.7. For the range of aspects ratios examined, it can be concluded that a correction for friction in the calculation of flow stress is unnecessary.     

Analysis of Deposits from Friction at Head-Tape Interfaces by Raman Spectroscopy

To demonstrate the applicability of Raman spectroscopy to the analysis of thin heterogeneous deposits on surfaces, a sample of brown stain or friction polymer produced on sendust (85% Fe, 9% Si, 6% Al) surfaces of a simulated read/write head by a magnetic oxide tape making prior contact with a copper, aluminum or nickel surface was investigated. Raman spectra identified the base film of the tape as poly(ethylene terephthalate) or PET, and the magnetically active layers as containing mostly γ-Fe₂O₃. Conversely, Raman spectra of brown slain showed it to consist mainly of Fe₃O₄ and organic compounds. More brown stain was produced after copper contact than after aluminum contact, but only organic deposits were produced after nickel contact in these tests. It is speculated on the basis of these spectral data that the inhibiting effect of nickel could be the result of catalytically produced organic materials, which change the tribology on the head surface and inhibit the formation of brown slain or friction polymer.     

Lubricant-Induced Spacing Increases at Slider–Disk Interfaces in Disk Drives

In this article, we explore the physical mechanisms for lubricant migration on recording head slider surfaces and how this migration leads to increased slider–disk spacing during disk drive operations. This is done using both a new experimental methodology, called the “droplet stress test,” and through simulation. In our simulations, we compare the air shear-induced lubricant migration modeled either as viscous flow of a continuum liquid film with zero slip or as wind driven slippage of molecules across the surface. The experimental data are best fitted using the viscous flow model to determine an effective viscosity for the sub-nanometer thick lubricant films. This effective viscosity tends to be somewhat less than the lubricant bulk viscosity due to air shear promoting the slippage of lubricant molecules across the surface. Our experimental results also indicate that the potential spacing increase from the pickup of disk lubricant on the slider is limited by the mobile fraction of the dewetting thickness of the lubricant film on the slider.     

Frictional performance of an O-ring type seal at the commencement of linear motion

Results of investigation into the relationship between friction in the O-ring type seal and gauge pressure at the commencement of linear motion of a shaft are presented and discussed. A number of different O-ring materials were studied and the lowest friction under dry conditions was found to be produced by a PTFE-encapsulated silicone seal. The effect of a number of lubricating fluids on friction in the seal was also studied and the results obtained are included.     

Frictional charging of fuel

A method is proposed for reducing the smoke content and toxicity of exhaust gases by frictional charging of motor fuel. Experimental data are presented.     

Frictional Voltammetry with Copper

This paper presents an experimental study correlating frictional behavior with in situ voltammetry for a unidirectional sliding contact between a hemispherical tipped alumina probe and a flat rotating copper counterface (maximum Hertzian contact pressure of 68 MPa and sliding speed of 10 mm/s). The contact was immersed in an aqueous 0.1 M Na₂CO₃ solution (pH ∼11) where the copper counterface acted as the working electrode in a potentiostat controlled three-electrode cell; a coiled Pt wire was used as the counter electrode and a saturated calomel electrode (SCE) as the reference. Clear and reproducible trends were found between friction coefficient and published data suggesting the onset of particular redox reactions, graphically presented in a frictional voltammetry plot. At anodic potentials primarily associated with the formation of copper(I) oxide (Cu₂O) (V vs SCE ∼−0.25), the measured friction coefficient was in the range μ ∼0.4–0.5. At cathodic potentials primarily associated with the formation of CuO, Cu(OH)₂, and CuCO₃ (V vs SCE ∼−0.10), the friction coefficient transitions to the range μ ∼0.7–1.0. At sustained cathodic potentials associated with reduction of the native copper oxide, Cu₂O, (V vs SCE ∼−0.65), the friction coefficient is observed to fluctuate between μ ∼0.2 and 0.5, arguably a result of exposure of bare copper due to non-uniform reduction (fractional coverage) of Cu₂O.     

不同接口对高效液相色谱－质谱分析甾体激素的影响

本文采用传送带接口和热喷雾接口，以８种甾体激素的混合标样，就不同接口对ＬＣ／ＭＳ的影响进行了评价。     

Collective Thomson scattering measurements with high frequency resolution at TEXTOR

We discuss the development and first results of a receiver system for the collective Thomson scattering (CTS) diagnostic at TEXTOR with frequency resolution in the megahertz range or better. The improved frequency resolution expands the diagnostic range and utility of CTS measurements in general and is a prerequisite for measurements of ion Bernstein wave signatures in CTS spectra. The first results from the new acquisition system are shown to be consistent with theory and with simultaneous measurements by the standard receiver system.