Mixed Lubrication Regime Research Articles

Tribological performance of avocado oil containing copper nanoparticles in mixed and boundary lubrication regime

PurposeThis paper aims to study the tribological performance of Cu nanoparticles mixed in avocado oil.Design/methodology/approachA Pin-on-Disc tribometer was used to determine the tribological performance of avocado oil as a lubricant as well as for measuring the effectiveness of Cu nanoparticles. Stribeck curve was generated with the base oil and the oil containing Cu nanoparticles. The nanoparticles are added in 0.5 wt. % and 1 wt. % concentration. The worn-out surfaces of aluminum alloy 6061 pins are explored by scanning electron microscopy (SEM).FindingsThe use of Cu nanoparticles led to a reduction in friction and wear. Coefficient of friction (COF) was found to be minimum at 1 wt. % concentration, whereas specific wear rate was minimum for 0.5 wt. % concentration. The film-formation capability of the Cu nanoparticles led to an overall improvement in the tribological properties of the base oil.Originality/valueExperiments are performed to evaluate the tribological performance of avocado oil using Cu nanoparticles. The results obtained herein suggest that avocado oil has a great potential to replace the conventional mineral oils in the field of industrial lubrication.

Effects of the micro surface texturing in lubricated non-conformal point contacts

Abstract This paper presents an experimental study on the effects of micro-textured surfaces on lubricated non-conformal point contacts. Thus, we focus on a regime poorly investigated in literature, where the contact area and the micro-holes have a comparable size. Tribological characterization are performed on three geometrical patterns, which are textured on stainless steel polished surfaces. Experiments are carried out on a rheometer, where a steel ball slides against the surface of the samples. These samples are tested with two different viscosities of the PAO (Poly-Alpha-Olefin)as a lubricant. Results show the change in the friction with respect to the sliding velocity under different lubrication regimes due to the stress, void ratio and two different kinematic viscosities of PAO. In particular, we show that, depending on the void ratio, a significant friction reduction or, on the contrary, a deterioration of the frictional performances can affect the boundary and mixed lubrication regimes. This is due to the simultaneous occurrence of two competing effects. One is related to the stress intensification, due to the presence of the micro-hole edges on the contact topography, which leads to a consequent increase in wear and friction. On the other hand, micro-texture may play a positive role in the friction optimization given the possibility, offered by the micro-holes, to entrap wear debris and, then, to preverse a smoother interface between the contacting pairs.

Modification of Alkali Lignin with Poly(Ethylene Glycol) Diglycidyl Ether to Be Used as a Thickener in Bio-Lubricant Formulations.

Considerable efforts are currently being made by the academic community and industry, aiming to develop environmentally friendly lubricants with suitable technical features for their performance. In this context, lignin could be considered a promising candidate to be used as a bio-sourced thickening agent to formulate eco-friendly lubricating greases. In this work, alkali lignin (AL) was chemically modified with poly(ethylene glycol) diglycidyl ether (PEGDE). Afterwards, the epoxidized lignin was properly dispersed in castor oil (CO) in order to obtain an oleogel for lubricant applications. The epoxidized lignins were characterized by means of epoxy index determination, thermogravimetric analysis (TGA) and Fourier transform infrared (FTIR) spectroscopy. The epoxide-functionalized lignin-based oleogels were analyzed from both rheological and tribological points of view. It was found that the viscosity, consistency and viscoelastic functions of these oleogels clearly increased with the epoxy index of the epoxide-modified lignin compound. Thermo-rheological characterization of these oleogels revealed a slight thermal dependence of the viscoelastic moduli below 100 °C, but a significant softening above that critical temperature. In general, these oleogels showed low values of the friction coefficient under the mixed lubrication regime as compared to the neat castor oil.

Numeric based low viscosity adiabatic thermo-tribological performance analysis of piston-skirt liner system lubrication at high engine speed

Abstract In conjunction with lubricant oil film dynamics, automotive piston's primary and secondary motion indicatively affect the engine noise and frictional losses (fuel economy). The escalation of oil film temperature due to adiabatic shear heating of low viscosity Newtonian lubricant oil film affects the load carrying capacity and viscosity of lubricating film and enhances friction losses, piston wear, and non-linear lateral motion of piston. In this study, a numerical model for piston dynamic and lubrication is presented, in which adiabatic heating effects are simulated in hydrodynamic mixed lubrication regime by coupling with 2D-energy equation. The thermo-hydrodynamic lubrication model is solved using finite difference formulation, while the piston dynamic model is solved using the modified Newton Raphson method. Furthermore, the effect of adiabatic shearing of lubricant and piston skirt design parameters on piston dynamics and lubrication performance are analyzed. Results show that the adiabatic heating significantly affects piston lateral motion and lubrication performance. Lubrication analysis is very beneficial in the improvement of engine design of piston skirt-liner and can help reduce the friction losses.

Ionic Liquid Additives for Mixed and Elastohydrodynamic Lubrication

ABSTRACTIonic liquids (ILs), both as pure lubricants and as lubricant additives, have been demonstrated extensively to exhibit excellent tribological performance in terms of friction and wear reduction in the boundary lubrication (BL) regime. Because engineering contacts experience boundary and mixed as well as full film lubrication depending on operating conditions, it is crucial to examine whether lubrication regimes other the BL regime can also benefit from the use of ILs. The objective of this work is to investigate the tribological performance of IL additives in the mixed lubrication (ML) and the elastohydrodynamic lubrication (EHL) regimes. Polyethylene glycol (PEG) was used as the base fluid. ILs were synthesized in situ by dissolving lithium bis(trifluoromethanesulfonyl) imide (LiTFSI) in PEG. Friction and film thickness measurements were employed to investigate the effectiveness of IL additives at room temperature, 60°C, and 80°C at various loads and slide–roll ratios (SRRs). The effect of IL additives on the rheological behavior of PEG was also investigated. The EHL film thickness increases with increasing IL concentration. EHL friction is, however, only mildly affected by IL additives. In the ML regime, IL additives can reduce friction and metal wear compared to pure PEG in mild conditions. It is conjectured that IL forms sacrificial layers and protects the rubbing surfaces.

Superlubricity of 1-Ethyl-3-methylimidazolium trifluoromethanesulfonate Ionic Liquid Induced by Tribochemical Reactions.

The robust liquid superlubricity of a room-temperature ionic liquid induced by tribochemical reactions is explored in this study. Here, 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIM]TFS) could realize stable superlubricity (μ < 0.01) with water at the interfaces of Si3N4/SiO2. A superlow and steady friction coefficient of 0.002-0.004 could be achieved under neutral conditions (pH of 6.9 ± 0.1) after 600 s of running-in process. Various factors that could affect superlubricity were explored, including concentration of [EMIM]TFS, sliding speed, applied load, and volume of the lubricant. The results reveal that superlubricity can be achieved with [EMIM]TFS aqueous solution under a broad scope of conditions. The results of surface analysis show that a steady composite tribochemical layer comprising [EMIM]TFS, silica, ammonia-containing compounds, and sulfides was formed by tribochemical reactions between [EMIM]TFS and Si3N4 during the running-in period. The film thickness calculation reveals that the achieved superlubricity is in a mixed lubrication regime that comprises boundary lubrication and thin film lubrication. The superlubricity state is governed by a firm composite tribochemical layer, a molecular adsorption layer (electric double layer of [EMIM]TFS), and a fluid layer. The liquid superlubricity achieved by the ionic liquid is helpful for the development of new ionic liquids with superlubricity characteristics and is of great significance for scientific understanding as well as engineering applications.

Effect of degradation on tribological performance of engine lubricants at elevated temperatures

Abstract High operating temperatures in engines produce a thermal degradation promoting the oil oxidation that affects its tribological performance. In this paper, an alternative method is proposed to simulate engine oil degradation by mixing degraded oil with fresh oil to simulate a mileage. The simulated and real degraded engine oils were compared by infrared spectroscopy analysis. The dynamic viscosities of mixed oils were measured by a rheometer at 50 °C and 80 °C and a block-on-ring tribometer was used to evaluate their tribological performance under mixed lubrication regime at these two temperatures. The results showed that the simulated degradation is acceptable, the coefficient of friction decreased at higher temperature and reduced as the engine oil is close to its end of service life.

On the modelling of mixed lubrication of conformal contacts

Abstract When it comes to the numerical evaluation of tribological contacts operated in the boundary and mixed lubrication regime, the estimation of the asperity contact based on statistical methods is widespread. We have studied the effects and expressiveness of major aspects of statistical contact modelling including the choice of contact model, surface evaluation method and magnification in the process of surface acquisition. To this, friction experiments including two material combinations were conducted and compared with numerical results retrieved from a newly developed numerical framework in a commercial simulation software. The results point out the relevance of the numerical framework and visualize the effect of inappropriate surface acquisition respectively the expressiveness of the investigated evaluation methods and contact models.

Wear Prediction of Earth-Moving Machinery Joint Bearing via Correlation between Wear Coefficient and Film Parameter: Experimental Study

ABSTRACTA wear life prediction of a grease-lubricated bearing in earth moving machinery has to be made based on actual wear data for its accuracy, but much effort should be devoted to obtain the wear data from the actual bearing. Currently, manufacturers provide a very conservative maintenance guide to schedule the replacement of the bearings to prevent the mechanical failure due to wear, causing economic losses, and the market requires finding optimal maintenance cycles to reduce maintenance costs. In this study, an economical wear prediction methodology for the grease-lubricated bearing based on typical pin-on-disk (POD) tests is proposed. POD tests were performed under boundary and mixed lubrication regimes considering the actual operating conditions of the bearings. It was found that there is a linear relation between the lubrication film parameter and the wear coefficient in log-log scale. The amount of wear of actual bearings was estimated by the wear coefficient from the linear regression analysis based on the POD test. The wear tests were further performed with the actual bearing under two loading cases and then the lubrication film parameter and wear coefficient were calculated, respectively. The estimated amount of wear shows good agreement with the measurement of wear depth. This leads us to conclude that it is possible to economically predict the wear amount in an actual bearing from POD wear test results by analysis of the correlation between the wear coefficient and the lubrication film parameter.

Can a “pre-worn” bearing surface geometry reduce the wear of metal-on-metal hip replacements? – A numerical wear simulation study

Total Hip Replacement (THR) is generally a highly successful treatment for late stage hip joint diseases and wear, however, wear continues to be one of the major causes of metal-on-metal THR's failure. Hip replacements typically experience a two-stage wear; a higher initial wear rate in the beginning followed by a lower steady-state one with the surface profile changed. This alludes to the potential use of a cup with a non-spherical interior cavity with an initial geometry similar to a worn surface which may benefit from lower wear rate. In this paper wear is numerically simulated with a cup having a non-spherical geometry inspired by the initial stage of wear.A wear model was recently developed by the authors for the THR, which considered the lubricated contact in both elastohydrodynamic lubrication (EHL) and mixed lubrication regime, rather than a dry contact used in most of other studies of wear modelling in the academic literature. In this study the wear model has been updated by introducing the ‘λ ratio’ (the ratio of film thickness to surface roughness) and addressing the non-Newtonian shear-thinning properties of the synovial fluid. This wear model was able to describe the non-linear wear evolution process due to the change of worn profiles. Firstly the wear of a spherical hip joint was simulated until a steady-state wear rate is achieved. Then a non-spherical joint was proposed in which the cup bearing geometry was generated by the previously predicted worn profile from the spherical joint. At last the wear of this “pre-worn” hip bearing was simulated and compared to the spherical one. Approximately 40% reduction in the steady-state wear rate and 50% in the total accumulated wear has been observed in the non-spherical hip joint. This study presented a full numerical analysis of the relationship between lubrication, wear reduction and the geometry change, and quantitatively suggested the optimal geometry to reduce running-in wear.

Friction and Wear Reduction Mechanism of Polyalkylene Glycol-Based Engine Oils

ABSTRACT Polyalkylene glycols (PAG) have been explored as a possible base stock for engine oil formulation. The friction, wear, and load-carrying capacity of five different PAG chemistries were evaluated either as a base stock or as formulated oils in pure sliding and sliding-rolling conditions using various laboratory bench test rigs operating under boundary and mixed lubrication regimes. The results were compared against GF-5 SAE 5W-20 and a mineral-based oil. The wear surfaces were also characterized using various surface-sensitive techniques for analysis of tribofilms to understand the mechanism of friction reduction. The results indicated that PAG oils show lower friction/traction coefficients and improved load-carrying capability, depending on the formulation, than those of the GF-5 SAE 5W-20 and mineral-based oil. The adsorption of PAG molecules on the surface appeared to be responsible for the lower friction characteristics.

An experimental framework for determining wear in porous journal bearings operated in the mixed lubrication regime

Abstract We present an experimental and analysis workflow to characterize the tribological behavior and the wear resistance of porous journal bearing systems operating at high loads and small rotational speeds. Our approach consists of a laser-instrumented tribometer that allows parallel testing of five bearings, an experimental procedure that is optimized for producing sufficient wear during mixed lubrication operation while maintaining realistic operating conditions, as well as several methods to visualize and quantify bearing wear. The simultaneous testing of five bearings prevents outliers from distorting the results and yields a statistical estimation of the performance variations between nominally equivalent tribosystems. We showcase our approach by analyzing the influence of the bearing material and its porosity on mixed-lubrication friction and wear.

A Novel Surface Texture Shape for Directional Friction Control

An experimental study is presented to evaluate the influence of anisotropically shaped textures on the behaviour of sliding friction and sensitivity to sliding direction. The plate samples were textured with triangular sloped dimples using an ultrafast laser surface texturing technique. Reciprocating cylinder-on-plate tests were conducted with steel sliding pairs using mineral base oil as a lubricant to compare the tribological performance of reference non-textured specimen and dimpled samples. The dimples were designed with varying converging angles in the transverse y–z plane and top-view x–y plane. In this study, no dimple was fully covered in the contact area since the dimples size is much larger than the Hertzian line contact width. Stribeck style dynamic friction curves across boundary, mixed and hydrodynamic lubrication regimes were used to determine the benefit or antagonism of texturing. Observation of the directional friction effect of the anisotropic textures indicated that the converging shapes are beneficial for friction reduction, and the dimpled specimens have a lower friction coefficient particular under prevailing boundary lubrication conditions. It was also found that the real contact length variation rate is a major factor controlling the local friction response. The sloped bottoms of the textures produce effective converging wedge action to generate hydrodynamic pressure and contribute to the overall directional friction effects.

Synergy of two fatty acids as additives on lubricity of a nematic liquid crystal 5CB

AbstractIn examining the lubricity of a nematic liquid crystal (4‐pentyl‐4′‐cyanobiphenyl [5CB]), synergy of 2 fatty acids (stearic acid [C18] and behenic acid [C22]) as additives was observed. In “friction measurements” using a ball‐on‐plate‐type tribometer, the friction coefficient for the “mixed‐additive” lubricant (5CB‐C18‐C22) was lower than those for the “single‐additive” lubricants (5CB‐C18 and 5CB‐C22). This synergy mechanism was discussed with considering the results obtained in other tests: “orientation observations” in a stationary cell using polarising microscopy and “surface topography measurements” of adsorbed films using atomic force microscopy. As a result, it is believed that the microscopic patterned surface formed by the mixed additive strengthens the surface anchoring of 5CB, which leads to increase in the effective viscosity of 5CB and therefore to decrease in the macroscopic friction coefficient in the mixed lubrication regime.

Study of Dimple Effect on the Friction Characteristics of a Journal Bearing using Taguchi Method

The effect of producing dimples using chemically etched techniques or by machining process on the surface of a journal bearing bushing to reduce the friction using Taguchi method is investigated. The data used in the present analysis is based on the results obtained by the series of experiments conducted to study the dimples effect on the Stribeck curve. It is statistically proved that producing dimples on the bushing surface of a journal bearing has significant effect on the friction coefficient when used with light oils. Also it is seen that there is an interaction effect between speeds-load and load-dimples. Hence the interaction effect, which are usually neglected should be considered during actual experiments that significantly contributes in reducing the friction in mixed lubrication regime. The experiments, if were conducted after Taguchi method, then the number of experiments would have been reduced to half of the actual set of experiments that were essentially conducted.

Combined experimental and multiphase computational fluid dynamics analysis of surface textured journal bearings in mixed regime of lubrication

AbstractThis paper investigates the effect of surface texturing in a partial pad journal bearing through a series of controlled experiments at operating conditions, promoting mixed or boundary regimes of lubrication. Improvements to load carrying capacity are observed under certain operating conditions. A comprehensive computational finite volume multiphase fluid dynamics analysis, including vapour transport equation and modified finite‐size cavity Rayleigh‐Plesset model, is used to study the effect of indented surface textures in the microscale contact domain and within the individual textures themselves. The results show improved conditions with a textured journal through promotion of micro‐hydrodynamic effect, delaying the effect of lubricant rupture, thus extending the effective load bearing region. A very good agreement is obtained between measurements and predictions.

Experimental analysis of the hydrodynamic effect during start-up of fixed geometry thrust bearings

Abstract Most of the investigations on thrust bearings were performed under a steady-state condition and this regime is now well understood. Contrary to this, the behavior during the first period of start-up is sparsely studied. Start-ups of thrust bearings are critical periods where a mixed lubrication regime occurs. This regime can be detrimental for the lifetime of the system. It is usually mentioned in the literature that the textures act as micro reservoirs which can be beneficial to help lift-up during start-up. This is an intuitive hypothesis and there are no experimental results to prove it. This study uses an experimental approach in order to analyze the behavior of different kinds of thrust bearings with fixed geometry. It mainly focuses on the capability of textured thrust bearings to lift up during start-up. This study shows that only specific texture geometry can help the formation of a fluid film. The results are compared to those obtained with classical geometries like tapered land, pocketed and untextured flat-land thrust bearings. The original results show a relatively good ability of flat-land thrust bearings to generate a lift-up during the start-up period. An experimental device is used to measure, at high frequency, the friction torque as well as the electrical resistance across the fluid film to determine the lubrication regime. The comparative analysis allows us to explain the factors which modify the contact.

Modeling, analysis and constrained control of wet cone clutch systems: A synchromesh case study

This paper addresses the problem of controlling the regime of lubrication in sliding lubricated surfaces with the purpose of wear reduction and the lifetime increase of the friction lining material. To this end, the hydrodynamic, mixed, and boundary lubrication regimes in a cone clutch system are experimentally investigated and a piecewise linear approximation of the Stribeck curve is presented. The friction-induced temperature rise and its effect on viscosity reduction, wear, and dynamic uncertainty are discussed. A modified wear model of the cone clutch is proposed by taking into account the contributions of both the mixed and boundary lubrication regimes. Using the obtained wear model, the necessity of the operation in the mixed lubrication regime is shown, and the duration of the operation is determined. The dynamical model of the system is constructed and the friction-induced instability of the system in the mixed lubrication regime is investigated and discussed. The operation in the mixed lubrication regime together with the friction-induced instability reveals the need for a closed-loop control system. Employing the obtained dynamical, frictional, and wear models, a controller design method is developed to meet the objectives and satisfy the constraints. The proposed controller design method results in a piecewise affine (PWA) feedback law obtained by solving a set of linear programming (LP) problems offline. The resulting controller is easily implementable in real time. The robust stability of the proposed closed-loop control system is proved considering the uncertainties of the dynamical system. The robust performance of the PWA closed-loop control system in the presence of various disturbances is assessed through simulation and confirmed by experiments conducted on a test rig. The case study here is the engagement process of a synchromesh system in a clutchless automated manual transmission of an electric vehicle.

Aqueous Lubrication, Structure and Rheological Properties of Whey Protein Microgel Particles.

Aqueous lubrication has emerged as an active research area in recent years due to its prevalence in nature in biotribological contacts and its enormous technological soft-matter applications. In this study, we designed aqueous dispersions of biocompatible whey-protein microgel particles (WPM) (10-80 vol %) cross-linked via disulfide bonding and focused on understanding their rheological, structural and biotribological properties (smooth polydimethylsiloxane (PDMS) contacts, Ra < 50 nm, ball-on-disk set up). The WPM particles (Dh = 380 nm) displayed shear-thinning behavior and good lubricating performance in the plateau boundary as well as the mixed lubrication regimes. The WPM particles facilitated lubrication between bare hydrophobic PDMS surfaces (water contact angle 108°), leading to a 10-fold reduction in boundary friction force with increased volume fraction (ϕ ≥ 65%), largely attributed to the close packing-mediated layer of particles between the asperity contacts acting as "true surface-separators", hydrophobic moieties of WPM binding to the nonpolar surfaces, and particles employing a rolling mechanism analogous to "ball bearings", the latter supported by negligible change in size and microstructure of the WPM particles after tribology. An ultralow boundary friction coefficient, μ ≤ 0.03 was achieved using WPM between O2 plasma-treated hydrophilic PDMS contacts coated with bovine submaxillary mucin (water contact angle 47°), and electron micrographs revealed that the WPM particles spread effectively as a layer of particles even at low ϕ∼ 10%, forming a lubricating load-bearing film that prevented the two surfaces from true adhesive contact. However, above an optimum volume fraction, μ increased in HL+BSM surfaces due to the interpenetration of particles that possibly impeded effective rolling, explaining the slight increase in friction. These effects are reflected in the highly shear thinning nature of the WPM dispersions themselves plus the tendency for the apparent viscosity to fall as dispersions are forced to very high volume fractions. The present work demonstrates a novel approach for providing ultralow friction in soft polymeric surfaces using proteinaceous microgel particles that satisfy both load bearing and kinematic requirements. These findings hold great potential for designing biocompatible particles for aqueous lubrication in numerous soft matter applications.

Mixed-lubrication analysis of misaligned bearing considering turbulence

Abstract Some bearings operate in mixed-lubrication regime due to the low rotational speed and the heavy load and misalign due to shaft's gravity. For large bearings, the maximum film thickness is much larger than minimum nominal film thickness and turbulence may locally develop. This paper proposes an approach to analyze misaligned mixed-lubrication bearing considering turbulence. Based on the average flow model proposed by Patir and Cheng and the Ng-Pan turbulence model, a generalized average Reynolds equation is derived. The calculation procedure is established by finite difference method. The results show that turbulence remarkably increases friction coefficient, slightly increases the minimum nominal film thickness, and decreases the transition speed from mixed-lubrication regime to hydrodynamic lubrication regime.