Mixed Lubrication Mode Research Articles

Inter-ply friction behaviour in the temperature assisted forming of magnesium/glass fibre reinforced thermoplastic polymer laminates

The paper investigates the metal/prepreg inter-ply friction behaviour in thermoplastic fibre metal laminates (FMLs) based on magnesium alloy and glass fibre-reinforced polymer (GFRP) under a range of parameters (normal pressure, relative sliding velocity, temperature) that are typical of forming processes conducted at elevated temperature on FMLs. The surface roughness of metal sheets was also varied and its influence on the inter-ply friction behaviour was evaluated. Pull-through tests with a stop-start controlling strategy were designed and carried out for identifying the inter-ply friction coefficient at varying conditions. To support the quantitative data from the tests microscopic observations were performed on the residues of the prepregs left on the tested metal sheets. The obtained results proved that the differences in the friction coefficient are strongly dependent on the process conditions and the metal surface roughness. The increase in the relative sliding displacement enables the transition from the hydrodynamic to the mixed lubrication mode according to the Stribeck theory, which further rises the friction coefficient. The inter-ply friction coefficient is observed to increase at decreasing temperature and normal pressure, and at increasing sliding velocity and metal surface roughness.

Tribomonitoring of the quality of aviation hydraulic oils according to lubricity and rheological indicators

The study proposes a diagnostic control method for assessing the quality of commercial batches of hydraulic oils based on the kinetics of changes in the thickness of lubricating layers, shear stresses of the lubricating material, and effective viscosity in tribotechnical contact. Timely and high-quality tribomonitoring of lubricants provides a perspective on their rational use and reduced wear of equipment parts. The developed methodology simulates the operation of gears in rolling conditions with a slip of 30% using a roller analogy. Samples of AMG-10 oil from two manufacturers were analyzed. For "Bora B" AMG-10 oil (sample 1) with gradients of the sliding speed of the lubricating layer in contact from 5.63·103 to 5.73·105 с-1, the effective viscosity is set at the level of 4249 and 5039 Pa·s at a bulk oil temperature of 20 and 100 oС, respectively, which indicates the resistance of oil components to destruction under conditions of increasing shear rate gradient. For AMG-10 oil (sample 2), the effective contact viscosity decreases by 1.53 times both at an oil temperature of 20 oС and at 100 oС and is 2764 Pa·s (at 20 oС) and 3309 Pa·s (at 100 oС), which indicates the destruction of the components of the lubricant. For "Bora B" AMG-10 oil, effective lubricating properties have been established both during the start-up period and at maximum revolutions in conditions of rolling with slipping. It was shown that at start-up, regardless of the temperature of the lubricant, the mixed lubrication mode dominates. At the maximum revolutions of the tested samples, the hydrodynamic lubrication mode dominates, which indicates the effective lubricating properties of the Bora B AMG-10 oil. According to the kinetics of changes in the rheological parameters of oils, it was established that the resistance of the lubricant's components to mechano-thermal destruction under non-stationary lubrication conditions contributes to the effective formation of a lubricating layer in contact with a high bearing capacity

Evaluation of operational properties of aviation oils by tribological parameters

The quality of aviation oils was evaluated online on the basis of their lubricating, antifriction, rheological and antiwear properties in the friction contact. The use of the software and hardware complex for evaluation of operational characteristics of triboelements is offered. Approbation of the proposed methodology was performed on aviation oils SM-9. The increase in antifriction properties of the “Bora B” SM-9 oil was established to be due to the formation of limiting adsorption layers of lubricant on friction-activated contact surfaces, which are characterized by low shear stresses of the lubricant, and their structuring provides high effective viscosity in the contact at a level of 5142 Pa.s. It was revealed that at start-up the lubricant temperature is 20 0С and the mixed lubrication mode prevails, but with increasing the lubricant temperature to 100 0С the elastic-hydrodynamic (contact-hydrodynamic) lubrication mode dominates, then at maximum rotation speed of friction pairs the hydrodynamic lubrication mode dominates, regardless of oil temperature, which indicates the effective separation of the contact surfaces due to the formation of a lubricating layer between them. Analysis of the specific work of friction in the friction contact showed that the instability of this parameter evidences to intensification of destructive processes in the near-surface layers of metal and reduction in its wear resistance. The decrease in wear resistance of the lagging surface in the conditions of rolling with sliding for all types of investigated oils is due to the different directions of the friction force vector in the contact. In the course of operation of friction pairs in nonstationary conditions, the softening of the surface metal layers occurs, which has a positive effect on the tribological processes in the contact. The practical significance of the work consists in developing a methodology of analysis of lubricants, which makes it possible to more accurately evaluate their performance and provide recommendations for the choice of lubricant for specific friction units.

Experimental characterization of the inter-ply shear behavior of dry and prepreg woven fabrics: Significance of mixed lubrication mode during thermoset composites processing

A custom-built test device was developed to measure the inter-ply shear resistance of dry carbon woven fabrics as well as prepregs at different temperatures, consolidation pressures, and forming rates typical to those used in autoclave processing. A quantitative understanding of the nature of the shear forces encountered during fabric consolidation was established, suggesting the strong dependence of these forces on the process conditions along with the dominance of a mixed lubrication regime. Among the most influential state variables, the resin viscosity and distribution of the resin on the prepreg surface, as well as the surface roughness of the reinforcing fibers could be identified.

The importance of the amount/thickness of die wall lubricant for UO2 pellets pressing

External lubrication is often used to complete compaction process of powder materials. The main goal of this method is generally to reduce the amount of admixed internal lubricant (Zinc stearate powder) within the raw material. The application of external lubricants enhances the density uniformity and the mechanical strength of the resulting compacts. This study investigates the effects of the external lubricant amount for UO2 powder compaction and the properties of the corresponding green pellets (corresponding to the compacts before sintering) without any admixed lubricant in the raw powder in order to evaluate the feasibility of this route in the case of nuclear powder. Results show that there is a quantity or number of layers from which the external lubricant applied on the die wall becomes detrimental to the friction index and the ejection force measured during the pressing cycle. The quality (surface defects, mechanical strength) of the green pellets can also be affected by the amount of lubricant. Thus the quantity and the thickness of the die wall lubricant must be optimized in order to assure an efficient mixed lubrication mode corresponding to the better lubrication mode in our study case.

Operational limit conditions of the spur gears in lubricated modes

The calculation of the gear teeth resistance, shows the using of a certain number of coefficients determined experimentally and which are accepted by the various international standards. However, this kind of calculation determines the gears by excess material and does not support the tribological parameters of operation. We propose in this work the support of these parameters, to determine the limit operation conditions of the spur gears, using the equivalent geometry. This is represented by two cylinders, which geometrically models of the contact between two teeth of a gear and whose lubrication is generally in mixed lubrication mode. The concept of Mc cool is used to determine the distribution of the load and the friction force, which are distributed in liquid (elastohydrodynamic) and solid domains and interact with each other. The phenomenon of interaction between the two domains is used, to predict the tribological limit conditions of operation. The proposed model is based on the resolution of elastohydrodynamic equations for the determination of load and friction as well as the deduction of mixed friction by tracing the Stribeck curve. This is calculated by the model of the decomposition of the patterns profile of rough surfaces in contacts. The results of non-dimensional calculations allow us to deduce the boundary conditions and can be adapted for any type of gear pair defined according to pre-established operating conditions.

A friction energy approach to quantifying lubrication under fretting sliding

AbstractThe problem of a proper lubrication under low‐speed small oscillatory movement can be a decisive factor for the reliability of various components. There is a need to characterise the lubricious behaviour of the interface under oil‐bath fretting wear conditions for ball bearing applications. Fast and reliable methods to quantify this behaviour for broad range of mechanical conditions are proposed and validated. Pure sliding reciprocation induces mixed lubrication mode. It was found that transient film profiles depend on the non‐Newtonian response of the oils and the type of motion. Running‐in period has a crucial importance for the tribofilm formation, and is a result of the interplay of the oil‐sliding surfaces interface and is directly connected with the total energy dissipated from the contact region. The stability of structured tribofilm in steady‐state period relies on the balance between the competitive processes: replenishment of the oil to the contact and ejection of the oil pending the oscillatory movement. The phenomenon of starvation was observed when the system was moved away from dynamical equilibrium and the growth of the dissipated energy was spotted. A proposed methodology provides the evaluation of the lubrication properties of the oil in a quantitative way. Copyright © 2010 John Wiley & Sons, Ltd.

An elastohydrodynamic lubrication (EHL) model of wear particle migration in an artificial hip joint

A full fluid ball-in-socket elastohydrodynamic lubrication (EHL) analysis of an artificial hip joint made of a metallic femoral head and ultra-high molecular weight polyethylene (UHMWPE) acetabular cup was considered. Since artificial hips operate in a mixed lubrication mode, wear occurs and wear particles lead to reduced hip lifetimes. This study involves simulating these particles within the lubrication regime. Hip deformation was compared to models employing finite element analysis and the spherical fast-Fourier transform technique. Particle modeling results were compared to suspension modeling experiments by other researchers. Results show a strong influence of lubricant fluid velocity on that of the wear particles.

Lubrication of the Human Ankle Joint in Walking

Human ankle joint lubrication in walking is analyzed. A biphasic mixture model is considered for articular cartilage (ideal interstitial fluid and elastic porous matrix that is transversely isotropic and inhomogeneous throughout its thickness). Synovial fluid is considered Newtonian. Its viscosity is due to the macromolecules of hyaluronic acid that are too large to enter the matrix pores. Due to the fluid pressure gradient water and small solutes pass through the matrix pores and across the articular surface in both directions. The effect of the time varying concentration of hyaluronic acid or of the synovial fluid viscosity on the synovial film thickness distribution is small and neglected in the model. Periodic sliding motion of the articular surfaces and periodic loading of the joint as encountered in walking are included in the analysis. Synovial fluid serves as a fluid lubricant. The model shows that soon after the onset of walking the normal human ankle joint works in a mixed lubrication mode (a combination of boundary and fluid-film lubrications). A protective gel layer formed in the gap due to the synovial fluid filtration by cartilage may serve as a boundary lubricant. The synovial gel layer is not guaranteed in the osteoarthritic case, and the rough sliding surfaces may get repeatedly into an intimate contact and wear off due to the reciprocating sliding motion.

Pad Topography, Contact Area and Hydrodynamic Lubrication in Chemical-Mechanical Polishing

AbstractMaterial removal during CMP occurs by the activation of slurry particles at contact points between pad summits and the wafer. When slurry is present and the wafer is sliding, contacts become lubricated. We present an analysis valid over the full range from static contact to hydroplaning that indicates that CMP usually operates in boundary or mixed lubrication mode at contacts and that the lubrication layer is nanometers thick. The results suggest that the sliding solid contact area is mainly responsible for the friction coefficient while both the solid contact and lubricated areas control the removal rate.

Contact–fluid interfacial slippage in hydrodynamic lubricated contacts

Contact–fluid interfacial slippage in hydrodynamic lubricated contacts

Computational simulation of wear rate and heat generation on a hip joint

A full fluid ball-in-socket elastohydrodynamic lubrication (EHL) analysis of an artificial hip joint made of a metallic femoral head and ultra-high molecular weight polyethylene (UHMWPE) acetabular cup was considered. Since artificial hips operate in a mixed lubrication mode, wear occurs and wear particles lead to reduced hip lifetimes. This study involves simulating these particles within the lubrication regime. Hip deformation was compared to models employing finite element analysis and the spherical fast-Fourier transform technique. Particle modeling results were compared to suspension modeling experiments by other researchers. Results show a strong influence of lubricant fluid velocity on that of the wear particles.

Mixed rheologies in elastohydrodynamic lubrication

This paper studies elastohydrodynamic lubrication (EHL) of line contacts for the slide‐roll ratios 0‐2 based on the assumptions of interfacial shear strength and interfacial slip. It is shown that the viscoelastic, viscoplastic and non‐continuum fluids distribute from the inlet zone to the Hertzian contact zone in order for a given operating condition when the load and rolling speed exceed critical values. For the rolling speed below the critical, the distributing fluids from the inlet zone to the Hertzian contact zone in order are viscoelastic and non‐continuum when the load exceeds a critical value. These show a multirheological behavior EHL film, formed in a contact, which may represent a mode of mixed lubrication. For this mode of lubrication, the fluid model should handle both inlet and Hertzian contact zones where the fluids are, respectively, continuum and non‐continuum. A new EHL analysis and theory, therefore needs to be established.

Characterization and modelling of piston ring‐cylinder tribosystem microtopography in lubricated contact

A study of piston ring‐cylinder tribological behaviour under mixed lubrication mode is usually based upon data regarding surface contact magnitudes, fluid film characteristics and resultant mixed friction forces. The present paper introduces a Fisher‐Pearson statistical model to describe elastic deformation of piston rings and liners’ asperities in order to derive more realistic functions that represent the aforementioned functional quantities considering real surfaces of used and worn liners. The hypothesis that the microtopography follows Gaussian law overestimates critical contact magnitudes such as: minimum film thickness‐surface separation distance, deformed asperities supported load, number of contacts and contact area. Increased time of use and wear of liners decreases transverse geometrical anisotropy. Correction contact integral functions in cases of used liners’ surfaces may be calculated through surface profile measurements. Theoretical and experimental results are in good agreement for the configuration considered.

Effects of lubricant rheology and additive chemistry in the wear of Si 3N 4 sliding on steel

The friction and wear characteristics of silicon nitride/steel sliding couples were examined with a pin-on-disk tribometer under geometry, speed and loads that simulate those encountered in the high-speed machining of steel. The tests were performed in dry sliding and under lubrication with pure paraffin and with paraffin containing 2% chlorinated paraffin or sulfurized olefin. In all conditions, the steel is transferred to the silicon nitride surface and the latter wears by fatigue-induced fracture during the removal of the transferred steel. In dry sliding, it was found that the wear of silicon nitride increases with the amount of frictional power dissipated. At high loads, however, high wear is caused by macroscopic fracture of the silicon. In the tribological conditions, chosen here to simulate those of high-speed cutting, lubricated sliding occurs in the mixed lubrication mode. Accordingly, pure paraffin causes a decrease in friction and wear that is more pronounced as the sliding speed increases or the load decreases. The EP additives used in this test reduce the friction coefficient further by boundary lubrication and decrease the wear rate of Si 3N 4 by another order of magnitude. Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for the analyses of the worn surfaces. Tribochemical reactions with the additives occurred on the steel surfaces only. Iron sulfides and chlorides were formed on the steel disk and chlorates and sulfates were found on the steel covering the ceramic disk. These differences are explained by the differing temperatures of pin and disk.

The lubricating ability of biomembrane models with dipalmitoyl phosphatidylcholine and gamma-globulin.

Two kinds of friction tests were conducted to investigate the lubricating effect of the injection of amphiphilies on the osteoarthritic joint. The effects of the addition of L alpha-dipalmitoyl phosphatidyl-choline (L alpha-DPPC) riposomes and gamma-globulin in a saline solution of sodium hyaluronate (HA) were evaluated through pendulum friction tests. The frictional characteristics of pig shoulder joints were confirmed to depend on the viscosity of the lubricants only in the physiologically low load condition and in the condition immediately after loading. Detergent (polyoxyethylene p-t-octylphenyl ether) was successfully used to remove adsorbed films from the articular surfaces. The friction coefficient of natural synovial joints was significantly increased in a mode of mixed lubrication with the HA solution of 0.2 g/dl by the treatment of the surface with the detergent. The addition of L alpha-DPPC riposomes or gamma-globulin significantly improved the boundary lubricating ability of the articular surfaces treated with the detergent, depending on the quantity of those additives. It appears that the L alpha-DPPC riposomes and gamma-globulin can form protective films on the articular surfaces like a biomembrane. Moreover, the reciprocating frictional behaviour in sliding pairs of pig articular cartilages and glass plates was studied in order to elucidate the tribological role of those constituents in the boundary lubricating film on the articular surface. Pig synovial fluid and water solutions of HA were used as lubricants. The synovial fluid had superior lubricating ability compared to the HA solution of equivalent viscosity under a physiologically high load condition. This fact seems to be responsible for the boundary lubricating ability of constituents other than hyaluronic acid. Langmuir-Blodgett (LB) films of L alpha-DPPC on the glass plate were kept at a low and stable friction coefficient, depending on the number of film layers. In conditions of mixed films with L alpha-DPPC and gamma-globulin, the frictional behaviour was improved by increasing the quantity of gamma-globulin. A model is proposed in which the effective adsorbed films are composed of proteins, phospholipids and other conjugated constituents on the articular surfaces to be accurate in describing the boundary lubricating mechanism. The mechanism is controlled by hydrophobic groups in those amphiphilies.

Effects of Electric Field on Lubricating Ability of Synovia Constituents. 2nd Report. Influence of Lubrication Mode.

The lubricating ability of synovia constituents and the effects of an electric field applied between sliding surfaces have been investigated in the wide range of lubrication regime using a roller-on-flat tester. Water solutions of sodium hyaluronate (HA, polysaccharide) with or without γ-globulin (protein) were used as lubricants. In friction tests for a conductive silicone rubber against a stainless steel or a titanium alloy (Ti-6Ai-4V, N+ ion implantation), the mixed lubrication ability of HA solution depended on the hydrophilic property of sliding material. Furthermore, marked increase of friction coefficient in the presence of γ-globulin was measured in the mixed lubrication mode. Since γ-globulin is water-insoluble, the molecule appeared to adsorb strongly onto the surface of hydro- phobic materials. These phenomena offered a high adhesion leading to an increase in friction for softer materials. The effects of applying alternative current (AC3V, f=10 Hz, simusoidal wave) on frictional behavior was observed in the wide range of lubrication modes. These results lead to one idea that the applying alternative current electric field plays roles in not only the promoting the adsorption of HA onto sliding surfaces due to the electric force but also the producing the weak-bond between the adsorbed HA or γ-globulin film and the sliding material due to a continuous changing the polarity of surfaces.

Lubricating Effect of Boundary Films Formed by Addition of Liposomes of Dipalmitoyl Phosphatidylcholine and .GAMMA.-Globulin in Synovial Joints.

The effect of addition of liposomes of dipalmitoyl phosphatidylcholine (DPPC) and γ-globulin on the natural joint lubrication was evaluated through pendulum friction tests. The frictional characteristics of pig shoulder joints were confirmed to depend on the viscosity of lubricants only in the physiologically low-load condition and immediately after the loading. A detergent (polyoxyethylene p-t-octylphenyl ether) was successfully used to remove adsorbed films from the articular surfaces. The friction coefficient of natural joints was significantly increased in mixed lubrication mode with a saline solution of sodium hyaluronate (0.2 g/dl) by surface treatment with the detergent. The addition of liposomes of Lα-DPPC or γ-globulin significantly improved the boundary lubricating ability of the articular surfaces treated with the detergent, depending on the quantity of the additives. It appears that the liposomes of Lα-DPPC and γ-globulin can form protective films on the articular surfaces in severe frictional condition.