Effect of some material and operating variables on scuffing

This paper summarizes the results of scuffing tests performed on AMS 6260 steel disks, covering three oils (a MIL-L-7808G oil, a MIL-L-23699A oil, and a straight mineral oil), two oil supply temperatures, a variety of sliding and sum velocities, and two modes of operating the test disks such that the potential failure sites on the disk surfaces either do or do not synchronize precisely in repeated cycles of operation. It is shown that, under otherwise comparable situations, (a) different oil-steel combinations allow the operation to penetrate by different degrees into the boundary lubrication regime before scuffing occurs, (b) an increase in the sliding velocity, at constant sum velocity, decreases the scuff failure load and the critical temperature, (c) an increase in the sum velocity, at constant sliding velocity, increases the scuff failure load and the critical temperature, (d) the effect of changing the sliding velocity or sum velocity, at a constant sliding-to-sum velocity ratio, depends on the balance of the opposing effects of sliding ans sum velocities at the particualar velocity ratio of interest, and (e) the scuff failure load and the critical temperature are markedly increased when the potential failure sites on the disk surfaces do not precisely synchronize on repeated cycles of operation.It is further demonstrated that the variations of the oil film thickness at scuffing, the coefficient of friction at scuffing, and the critical temperature with respect to all surface and operating variables correlate satisfactorily with a dimensionless parameter ξ_f.     

Influence of surface roughness on friction and scuffing behaviour of cast iron under sparse lubrication

Friction and scuffing behaviour of grey cast iron as influenced by the surface roughness under sparse lubrication conditions is studied. The studies are carried out on a three-shoe-on-disc machine under the conditions of parallel sliding. The experiments were conducted at three oil supply rates of 0.10, 0.22 and 0.36 μg/cm² per contact obtained through a mist oiling system and with specimens of cast iron shoes of different roughness values, Ra, between 0.04 and 2.0 μm against a 0.55% C steel disc. In a step load procedure, the friction torque at the end of each load step and the scuffing load are the major parameters measured. Results of friction and scuffing behaviour as a function of roughness and oil supply rates have been discussed.     

Tribological Behavior of Rice Bran Ceramics in a Vacuum Environment

ABSTRACT

In this study, we investigated the friction and wear of rice bran (RB) ceramics—hard porous carbon materials made from rice bran—in a vacuum environment. Sliding friction tests for RB ceramic pin–RB ceramic disk contact were performed using a pin-on-disk-type friction tester installed in a vacuum chamber. The ambient pressure was controlled at 0.02, 0.6, 30, and 10⁵ Pa (i.e., atmospheric pressure). The normal load was 0.49 or 2.94 N, the sliding velocity was 0.01 or 0.1 m/s, and the number of friction cycles was 50,000. The friction coefficient tended to decrease with decreasing ambient pressure for all combinations of normal load and sliding velocity; by contrast, the specific wear rate of the RB ceramic pin and disk specimens tended to increase with decreasing ambient pressure. The friction coefficient exhibited a low value of 0.05 or less at 0.02 Pa. The results suggested that the reduced surface roughness and graphitization of the sliding surface of the RB ceramic pin and disk due to induced friction, as well as the increased ratio between the partial pressure of water vapor and the ambient pressure, are related to the reduction in the friction of RB ceramic–RB ceramic dry sliding contact under vacuum conditions.     

Combined effects of surface roughness and viscosity variation due to additives on long journal bearing

Abstract

The combined effects of surface roughness and viscosity variation due to additives on long journal bearing are analytically studied. The variation in viscosity along the film thickness is considered. The presence of solid particles in the lubricant is an increased effective viscosity, which increases the load carrying capacity and decreases the frictional coefficient, whereas the viscosity variation tends to decrease both the load carrying capacity and coefficient of friction for non-micropolar fluid case. The modified Reynolds type equation for surface roughness has been derived on the basis of Eringen’s micropolar fluid theory. The generalised stochastic random variable with non-zero mean, variance and skewness is assumed to mathematically model the surface roughness on the bearing surface. Numerical results were obtained for the fluid film pressure, load carrying capacity and the coefficient of friction. It is observed that the combined effect is to increase the load carrying capacity and to decrease the coefficient of friction, which improves the performance of the bearing.     

Effects of surface roughness and surface films on contact resistance

An analysis of the electrical contact resistance between two metals with a tarnish film was carried out assuming the asperities to be represented by randomly distributed cones with base angles which vary with the surface roughness, and assuming that the radius of the broken area of the film at the interface of each contact asperity is constant beyond a critical depth of penetration of an asperity.The validity of the proposed theory was confirmed by experimental data of the electrical contact resistance between a silver cone and a silver flat on which carbon films were deposited, and between a silver flat with an Ag₂S tarnish and a palladium flat without tarnish. Comparison of theoretical and experimental data shows that the critical depth of penetration U_e of an asperity varies mainly with the surface roughness, the thickness of the tarnish film and the amount of plastic deformation of the contact asperities with films, i.e.$\text{U}{}_{\mathrm{e}}\text{σ}\text{= kR}{}^{\mathrm{j}}{}_{\max }$, where R_max and σ are the maximum height and the standard deviation (r.m.s. roughness) of the profile ordinates, and j and k are constants dependent on the type of finish, the thickness of the film and the difference in the hardness of the mating surfaces.     

滑动速度对油润滑表面胶合磨损的影响

在油润滑条件下,钢对钢摩擦副的胶合摩损不仅取决于润滑油膜是否破裂,而且取决于在摩擦表面上化学反应膜的形成情况。本文研究了在油润滑条件下滑动速度对钢摩擦副胶合的影响。在低滑动速度下摩擦表面易于形成反应膜,油膜破裂后并不直接发生胶合。胶合发生在高温、高摩擦系数的恶劣条件下。在高滑动速度下油膜破裂后很容易发生胶合,发生胶合前的表面温度和摩擦系数都比较低。     

Experimental Study on the Friction Characteristics of Lasertex Steel Sheets During Metal Forming Process

The surface of steel sheets used in the metal-forming process discussed in this article was textured by a laser-ablation technique. Differently shaped craters are formed in a patterned structure on the steel surface by controlling the pulsed-laser power density, pulse-repetition rate, and pulse duration. Lasertex sheets formed by this process have unique friction characteristics because of the uniform surface roughness and valley-biased topography. The friction of lasertex sheets was studied using a metal-forming bench test rig. Influencing factors, including surface roughness and sliding velocity, were studied under lubricated conditions. The friction of lasertex sheets was compared with that of shot-blasted sheets. The results showed that the coefficient of friction of the lasertex sheet under dry friction decreases with an increase in surface roughness and changes little with varying sliding velocity. With lubrication, the coefficient of friction of the lasertex sheet rises with an increase in surface roughness and decreases with an increase in sliding velocity. Lasertex sheets were found to have lower friction coefficients than shot-blasted sheets over the rage of surface roughness and sliding velocity investigated.     

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN_x) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.     

Tribology of ultra-high molecular weight polyethylene disks molded at different temperatures

Tribological characteristics of ultra-high molecular weight polyethylene (UHMW-PE) disks molded at 130–190 °C were studied. The highest crystallinity was obtained for the sheet molded at 130 °C, but crystallinity decreased with increasing molding temperature. Beyond 150 °C, the resultant crystallinity reached a constant level. The dynamic friction coefficients of these UHMW-PE disks were measured using a ball-on-disk friction tester. The friction coefficient decreased with increasing number of rotations in the early stage of the measurement, and achieved at an equilibrium level, independent of the molding temperature. The steady-state friction coefficient was 0.04 for the disk molded at 130 °C and increased with increasing molding temperature. The disks molded at 150–190 °C always had a steady-state friction coefficient of 0.065. The surface deformation of each disk was evaluated from the observation of the resultant wear track. Analyzing the relationship between the above friction coefficient and width of the wear track enabled us to interpret the tribological mechanism generated in this study.     

Effects of entraining velocity of lubricant and sliding velocity on friction behavior in stainless steel sheet rolling

A series of experiments was carried out using a rolling-type tribometer to investigate the effects on friction behavior of the entraining velocity of the lubricant at the inlet to the contact zone (V) and sliding velocity during deformation (ΔV). Experiments with stainless steel sheets of two different surface roughnesses showed that the variations in the friction coefficient with entraining velocity V and sliding velocity ΔV are largely dependent on the initial surface texture of the workpiece. For a smooth workpiece, the friction coefficient decreases with increasing sliding velocity ΔV but keeps almost constant with increasing entraining velocity V. However, for a rough workpiece, the friction coefficient initially decreases slowly and increases largely with increasing sliding velocity ΔV or decreasing entraining velocity V. Observation of the rolled surface for a smooth workpiece shows that, with increasing entraining velocity V, the slip band becomes more marked, and with increasing sliding velocity ΔV, the rubbed portions become more conspicuous. For a rough workpiece, galling occurs at high sliding velocity ΔV. The critical condition for galling outbreak is shown on the V-ΔV graph. The galling outbreak process is observed by interrupting the rolling process.     

Factors Influencing the Abrasion Resistance of Thin-Film Magnetic Media

Abrasion testing was performed on DC sputtered hydrogenated carbon films on commercial thin-film disks processed under several conditions. Four film characteristics were found to influence the abrasion resistance of the disk, i.e., roughness, film adhesion, carbon toughness, and coefficient of friction. It was found that both fine-scale surface topography from the sputtered layer and substrate texture degraded abrasion resistance. Excellent abrasion resistance was observed for carbon films as thin as 10 nm on polished substrates when the magnetic film topography was minimized. Film adhesion was degraded by exposure to temperature and humidity. Hydrogen incorporation into the carbon films reduced film hardness which degraded abrasion resistance.     

Investigation of adhesive and frictional behavior of GeSbTe films with AFM/FFM

Frictional force microscope (FFM) was used to investigate the nanoscale frictional behavior of GeSbTe films deposited by magnetron sputtering. The effects of relative humidity, scanning velocity and surface roughness on friction were taken into account. Besides, the frictional behavior of GeSbTe films with different compositions was analyzed. Experimental results show that the coefficient of friction of GeSbTe films is almost independent of scanning velocity, while the frictional force decreases with increasing velocity. Both the relationship of friction vs. normal load and that of friction vs. RMS keep relatively linear, and the coefficient of friction increases with the increase in RMS. The influence of humidity on adhesion between the tip and the GeSb₂Te₄ film is more significant than that between the tip and the Ge₂Sb₂Te₅ film.     

Signature Response for Friction and Wear

An experimental study of signature response for friction and wear is carried out in a triplane ball-on-peg machine under varying load conditions. Photographs of worn roller test specimens at critical temperatures were taken to determine the nature of the wear surface under boundary lubricated conditions. Acoustic response in the beginning and at the point of scuffing has demonstrated a definite trend which follows a similar pattern as that of friction. The experiment was repeated with a combination of steel and brass and with steel and mild steel using as the lubricants hexadecane, MoS₂ and light stock oil. The scuffing load was accompanied by severe wear of the surface. A critical value for friction and that of acoustic response was observed with variations in temperature. Results obtained contribute to the assesment in the change in friction and wear values on sliding surfaces. This help comes from a knowledge of acoustic emission and the known condition of the surface and both components of the pair. The findings of these experimental results and the mode of signature response provides a good scope for further investigations aimed at understanding the mechanism and establishing a method for diagnostic maintenance.     

Effects of static loading on surface parameters

The deformation of a roughened surface by a harder flat surface was calculated from digitised surface records taken with a relocation profilometer after each increment of load. The maximum normal pressure applied was 4.5 times the indentation hardness of the softer material. It was found that the effective surface hardness and topographical parameters were load dependent and consequently existing methods for predicting the real areas of contact were inapplicable at high loads corresponding to $\text{P}\text{M}{}_0\text{> 10}{}^{\mathrm{?2}}$.     

The Heat Transfer Coefficient and Lubricated Contact Temperature

This paper is one of a series dealing with the surface temperature conditions in disks and gears. It attempts to apply and extend the theory proposed in an earlier paper which predicted the behavior of the surface temperature of machine elements subjected to repeated frictional contacts. In considering the critical temperature hypothesis proposed by Blok as a criterion for failure by scuffing, it is essential that the operating surface temperature is taken into account in addition to the well known “flash temperature.” There have been many papers referrinq to this “flash temperature,” but little is known of the factors which affect the operating surface temperature, commonly called the bulk temperature. This paper demonstrates that the bulk temperature growth under loaded conditions can be predicted theoretically, and that by observing the behavior of the surface temperature the heat transfer coefficient and friction conditions can be calculated. Finally full details of the thermocouple used by the authors in their tests are given.     

Surface roughness and deformation of contact asperities between a rough and a flat surface

Conical asperity models with four kinds of apex angle and sandblasted surfaces were separately deformed by being pressed against a relatively hard flat surface. The real area of contact for the work hardened asperity models increased with decreasing apex angle and that for the sandblasted surfaces also increased with increasing surface roughness. The number of contact points and the separation between a sandblasted and a harder flat surface over the pressure range $\text{8.2 × 10}{}^{\mathrm{?3}}\text{≦}\text{W}\text{L}{}_{\mathrm{x}}\text{L}{}_{\mathrm{y}}\text{p}{}_{\mathrm{m}}\text{≦ 4.4 × 10}{}^{\mathrm{?2}}$ were also measured for comparison with the theoretical results. The experimental results for the sandblasted surfaces agreed with the theory. It is concluded that the real area, the number of contact points and the separation are influenced by the roughness of the softer surface.     

Influence of surface texture and roughness parameters on friction and transfer layer formation during sliding of aluminium pin on steel plate

Surface texture of harder mating surfaces plays an important role during sliding against softer materials and hence the importance of characterizing the surfaces in terms of roughness parameters. In the present investigation, basic studies were conducted using inclined pin-on-plate sliding tester to understand the surface texture effect of hard surfaces on coefficient of friction and transfer layer formation. A tribological couple made of a super purity aluminium pin against steel plate was used in the tests. Two surface parameters of steel plates, namely roughness and texture, were varied in the tests. It was observed that the transfer layer formation and the coefficient of friction along with its two components, namely, the adhesion and plowing, are controlled by the surface texture and are independent of surface roughness (R_a). Among the various surface roughness parameters, the average or the mean slope of the profile was found to explain the variations best. Under lubricated conditions, stick–slip phenomena was observed, the amplitude of which depends on the plowing component of friction. The presence of stick–slip motion under lubricated conditions could be attributed to the molecular deformation of the lubricant component confined between asperities.     

The Effect of Lubricant Traction on Scuffing

This paper presents the results of a disc machine gear simulation investigating the influence of lubricant traction characteristics and formulation on the load at which scuffing occurs. Scuffing theories in general link the onset of scuffing to the amount of heat generated in the contact and the authors hypothesized that reduced heat generation with low traction lubricants should lead to an increase in scuffing load. The study compared low traction PAO-based lubricants with mineral oils in basestock, antiwear and EP formulations and at both high (>6) and moderate (approximately 1.2) specific film thickness, λ. At λ > 6, the benefits of the synthetics over their mineral counterparts ranged from 25 percent to 220 percent and at λ ? 1.2, the benefits were a uniform 40 percent. It was particularly interesting to observe that the antiwear PAO-based oil gave a similar scuff load per unit contact width to an EP mineral gear oil. In addition, it was shown that scuffing load decreased with increasing traction coefficient to the power of approximately ? 1.85, close to the ?2.00 power predicted by the frictional power intensity concept. The agreement with flash temperature theory, with a predicted power of ?1.33, was less close.     

Some open questions in boundary lubrication

Bowden and Leben, in 1938, showed that there was a criticaltemperature limit for smooth boundary friction. This, Frewingshowed in 1942, could be analyzed by the van 't Hoff isochore.Next, the 150°C critical temperature of mineral oil was foundto be due to its surfactant component, which also followed thevan 't Hoff rule. This temperature was the same as Kelley andLemanski's 300°F limit for involute gears. Research needs inboundary lubrication are: the theory of boundary friction, thecause of the Borsoff effect, the fraction of vacant adsorptionsites at scuffing, the reason for chain matching and how load andsurface roughness alter the critical temperatures. Whenphysisorption changes toirreversible chemisorption, a micron-sized thick film forms,giving a thick boundary lubricating film. A mercury displacementrig can be used to show if and when such a thick film occurs onmetal surfaces. We have developed a high frequency rig (HFR) withreciprocating motions to study the temperature ranges that thisthick lubricating film is formed. Another issue is that decidingif the absence of an organic layer covering the asperities orDyson's failure mechanism of the EHL film is the effective causeof scuffing, is still very much an open question.     

Influence of roughness parameters and surface texture on friction during sliding of pure lead over 080 M40 steel

In the present investigation, tests were conducted on a tribological couple made of cylindrical lead pin with spherical tip against 080 M40 steel plates of different textures with varying roughness under both dry and lubricated conditions using an inclined pin-on-plate sliding tester. Surface roughness parameters of the steel plates were measured using optical profilometer. The morphologies of the worn surfaces of the pins and the formation of transfer layer on the counter surfaces were observed using a scanning electron microscope. It was observed that the coefficient of friction and the formation of transfer layer depend primarily on the surface texture of hard surfaces. A newly formulated non-dimensional hybrid roughness parameter called ‘ξ’ (a product of number of peaks and maximum profile peak height) of the tool surface plays an important role in determining the frictional behaviour of the surfaces studied. The effect of surfaces texture on coefficient of friction was attributed to the variation of plowing component of friction, which in turn depends on the roughness parameter ‘ξ’.