Mixed Lubrication Regime Research Articles

Application of the electrical impedance method to steel/steel EHD point contacts

The authors have previously developed the electrical impedance method which simultaneously measure the thickness and breakdown ratio of oil films in EHD (elastohydrodynamic) contacts. Furthermore, using a ball-on-disc-type apparatus, the authors have also confirmed that the accuracy of oil film measurement by the developed method is comparable to that of optical interferometry (i.e., glass-steel contacts). In this study, we discussed the experimental results obtained by applying the electrical impedance method to steel-steel contacts using a steel disc instead of a glass disc. It was found that under the mixed lubrication regime where wear occurs, the oil film thickness obtained by this method is calculated to be significantly thicker than the theoretical value. This can be attributed to the increase in the apparent dielectric constant of the lubricant due to wear particles in the lubricant.

Sliding-Induced Rehydration in Hydrogels for Restoring Lubrication and Anticreeping Capability.

Fluid exudation in cartilage under normal loading can be counteracted by a sliding-induced rehydration phenomenon, which has a hydrodynamic origin related to a wedge effect at the contact inlet. Similar to cartilage, hydrogels also exhibit tribological rehydration properties, and we mimic this phenomenon to restore hydration lubrication and overcome creeping. It occurs within a specific velocity range and is mainly dependent on the applied load and hydrogel network structures. Crucially, a certain velocity in the mixed lubrication regime can produce a hydrodynamic pressure peak at the wedge and drive the rehydration inflow to overcome the extrusion. At lower sliding velocities in the boundary lubrication regime, inflows are insufficient to counteract fluid exudation, whereas at higher velocities in the hydrodynamic lubrication regime, the inlet wedge effect would diminish. These results suggest that tribological rehydration offers a novel approach to enhancing load-bearing capacity and maintaining lubrication in the hydrogels.

Solid/liquid hybrid lubrication behaviors of amorphous carbon film coupling with nonpolar and polar base oils

The amorphous carbon film (a-C) has aroused intensive interest in solid-oil hybrid lubrication design due to its low friction, high wear resistance, and good chemical inertness. However, the solid/liquid hybrid lubrication behaviors and mechanisms of a-C films when coupled with nonpolar and polar oils of similar viscosity are still insufficiently understood and ambiguous to date. In this study, we compared the lubrication properties of nonpolar paraffin oil and polar polyether oils with different molecular structures when combined with a-C film. The results demonstrated that the lubrication performance of polyether oils coupled with a-C film surpassed that of paraffin oil across various lubrication regimes. Notably, the stable superlubricity (friction coefficient<0.01) was achieved under mixed lubrication regime on the a-C film when coupled with polyethylene glycol (PEG) oil after a prolonged sliding duration. The tribo-induced formation of carbon quantum dots (CQDs) at the steel/a-C friction interface lubricated by PEG oils was firstly observed. The hydroxylation of the counter ball surface and the fabrication of CQDs might jointly contribute to the achievement of the robust superlubricity of the PEG oil/a-C film hybrid lubrication system.

Non-Polar Chain-Enabled Suspension of Carbon Nanoparticles in Base Oil

The transition to electric vehicles (EVs) has introduced new challenges in lubrication, demanding innovative solutions to ensure consistent performance. One promising approach is the use of nanoparticle additives, which have the potential to improve lubrication performance significantly. However, achieving a stable suspension of these nanoparticles in lubricating oils remains a critical challenge, as suspension stability is essential for maintaining consistent performance and maximizing the benefits of these advanced additives. In this study, carbon nanoparticles (CNPs) were modified with dodecylamine (DDA) to achieve stable suspension in nonpolar fluids. The successful functionalization was confirmed by the FTIR results, which showed characteristic peaks of various bonding. The suspension stability tests demonstrated that DDA-CNPs remained suspended for over 60 days in the Polyalphaolefin (PAO) oil, whereas unmodified CNPs were sedimented within 3–7 days. The rheological behavior was measured under different shear rates and temperatures. Viscosity measurements indicated that DDA-CNPs maintained a lower value compared to base PAO. The lubricants’ friction coefficient (COF) was also determined under various speeds and loads. The addition of DDA-CNPs at a concentration of 0.05 wt.% resulted in a significant reduction in COF, with values decreasing by 26% compared to base PAO oil under a load of 1 N. Additionally, the COF for DDA-CNPs was consistently lower than that of PAO, with reductions ranging from 15% to 18% across all tested speeds. The Stribeck curve further highlighted the improved performance of DDA-CNPs across boundary, mixed, and hydrodynamic lubrication regimes. These findings suggest that DDA-CNPs significantly improve the lubrication performance of PAO oil, making them suitable for advanced lubrication applications in automotive and industrial systems.

Predicting resistance of bearing steels to surface initiated fatigue: Application to hybrid contact

The constant demand for improved energy efficiency in lubricated rolling/ sliding contacts, such as in rolling bearings, is generally addressed through increased power density and the use of low-viscosity (i.e., thin-film) lubricants. In these conditions, mixed lubrication regime can dominate with intensive asperity interactions between two contacting surfaces, thus increasing the risk of surface-initiated fatigue (e.g., in the form of micropitting). In the present study, the influence of steel mechanical properties on the micropitting accumulation in a hybrid contact (steel versus Si3N4 ceramics of different roughness) has been experimentally and theoretically investigated. It was found that higher roughness of Si3N4 surface could increase the risk of micropitting on the steel component. However, despite formation of some mild micropitting, the bearing steels tested here show good performance. For better understanding of the relationship between the mechanical properties of bearing steels and micropitting resistance, a new theoretical framework is introduced. The new micropitting model, based on the finding that the nucleation of a micropit is driven by (i) local accumulation of the plastic strain in mixed rough contact, and (ii) reduction of the total energy by debonding of the plastically deformed zone is introduced. The new micropitting model was shown to match with experiments across the range of test conditions. Micropitting performance maps based on the mechanical properties of steels (yield strength, work hardening and fracture toughness) have been obtained. The new theory has implications not only for predicting micropitting but also for predicting adhesive wear resistance of different steels.

Evaluation of surface texturing on chrome-coated cylinder liners via deterministic mixed lubrication simulation

This study investigates the mixed lubrication performance of various surface texture configurations in the piston ring/cylinder liner conjunction of a two-stroke internal combustion engine using a deterministic mixed lubrication model. The numerical model simultaneously solves the Reynolds equation with mass-conserving cavitation to calculate inter-asperity hydrodynamic pressures and an elastic, perfectly plastic, rough contact model to determine contact pressures at each asperity interaction. Gaussian Mixture Model clustering was employed to enhance surface characterization. The deterministic simulation approach considers the full-scale representation of the cylinder liner topography to accurately capture the influence of surface features on the hydrodynamic support and friction under mixed lubrication conditions. The investigated cylinder liners were initially hard-chrome-coated and honed, resulting in a stochastic arrangement of surface pores, and then deterministic patterns of surface pockets were created by micro electrodischarge machining (EDM). Surface measurements were performed using laser interferometry, providing input for the mixed lubrication simulations. The study also explored the virtual removal of ridges formed around the pockets by the EDM technique. Key findings indicate that the stochastic texture outperformed the hybrid texture (stochastic + deterministic) in the boundary and mixed lubrication regimes, showing higher hydrodynamic support at low separations but increased hydrodynamic shear stresses at higher speeds. Conversely, deterministic textures exhibited a significant decrease in average hydrodynamic shear stress at high velocities. These results highlight the critical role of surface texture in tribological behavior and suggest that localized textures on cylinder liners can potentially optimize engine performance. The study recommends further exploration of a broader range of texture geometries, densities, and distribution patterns to enhance engine design strategies.

Towards direct superlubricity and superlow wear via amino modification of polyhydroxy alcohol solutions

Friction remains as the primary mode of energy dissipation and components wear, and achieving superlubricity shows high promise in energy conservation and lifetime wear protection. The results in this work demonstrate that direct superlubricity combined with superlow wear can be realized for steel/Si3N4 contacts on engineering scale when polyhydroxy alcohol solution was selectively modified by amino group. Macroscopic direct superlubricity occurs because 3-amino-1,2-propanediol molecules at the friction interface could be induced to rotate and adsorb vertically on the friction surface, forming in-situ thick and dense molecular films to passivate the asperity contacts. Furthermore, amino modification is also conducive to improving the lubrication state from boundary to mixed lubrication regime by strengthening the intermolecular hydrogen bonding interaction, presenting enhanced load-bearing capability and reduced direct solid asperity contacts. Thus, direct superlow average friction of 0.01 combined with superlow wear are achieved simultaneously. The design principle of direct superlubricity and superlow wear in this work indeed offers an effective strategy to fundamentally improve energy efficiency and provide lifetime wear protection for moving mechanical assemblies.

Controlling stick–slip in low-speed motion with a lifting force of magnetic fluid

Stick–slip is a standard friction-induced self-excited vibration that usually occurs in the boundary or mixed lubrication regimes. Broadening of the hydrodynamic lubrication regime is conducive to suppressing stick–slip motion. In this paper, the load carrying capacity of a magnetic fluid (MF) film in the presence of a magnetic field is derived based on the modified Reynolds equation. An additional lifting force produced by MF under the magnet was applied between the tribopairs to achieve the full fluid lubrication. Thus, the stick–slip is expected to be inhibited in a lower speed scope. The effect of magnet thickness on the lifting force is investigated experimentally and theoretically. Special attention is given to the influence of the lifting force on the friction and the critical transition speed of the hydrodynamic lubrication regime. Results demonstrate that the lifting force increases with the increment of the magnet thickness. The presence of the additional lifting force expands the hydrodynamic lubrication and makes the critical transition speed move left, as shown by the friction transitions on the Stribeck curve. Therefore, stick–slip motion can be suppressed at a lower sliding speed. Such beneficial effects are more pronounced in thicker magnets. It can be confirmed that, so long as the lifting force is higher than the normal load, the friction will invariably operate in the full film lubrication and the stick-slip motion may be eliminated theoretically.

The role of adhesion on soft lubrication: A new theory

Recent experiments reveal that adhesive interactions can play a key role in causing surface instability in soft lubrication. Instances of instability include fluid entrapment in isolated pockets upon a soft sphere's normal contact with a hard substrate and surface wrinkling of a soft substrate as a hard sphere slides across it. These phenomena underscore a substantial distinction between hard and soft lubrication. They are of paramount importance from a fundamental standpoint, providing an entirely new explanation for the transition mechanism from elasto-hydrodynamic to the mixed lubrication regimes. Here, we introduce a new theory to elucidate these observations. Our theory modifies the Reynolds elasto-hydrodynamic equation by incorporating adhesive interaction across the fluid layer, investigating the interplay between adhesion, fluid flow and elastic instability. Our analysis proposes the addition of a new dimensionless parameter in lubrication theory, that compares the stiffness of the adhesive interaction to that of the substrate. When this parameter exceeds unity, the soft solid surface exhibits instability to small perturbations in its shape. In mathematical terms, the Reynolds equation undergoes a transition from a nonlinear diffusion equation to a nonlinear wave equation at this critical point. Post-transition, the diffusivity of the nonlinear diffusion equation turns negative, rendering the problem ill-posed. We investigate the transition using the method of characteristics and present an exact analytic solution. This solution offers insights into the occurrence of a vanishing liquid film thickness at specific locations, resulting in dry contact—initiating transition to mixed lubrication.

Thermal accumulation effects modelling of investigate radial shaft seal wear and lubrication transition mechanism

A radial shaft seal (RSS) model is established to reveal mixed thermal lubrication (MTL) and wear performance at speeds ranging from 3000 r/min to 7000 r/min. The model takes into account the initial temperatures of the oil medium and the running time of the RSS to investigate the transition limit of lubrication. Then a thermal fluid-solid method was developed to simulate the lubrication performance of RSS, which shows lubrication transition boundaries through rough micro convex bodies for wear deformation. The RSS deforms wedge-shaped during the wear process and a localized fluid pressure is created on the convex bodies. Considering the wear deformation of the convex bodies due to the wedge effect, the micro performance of the RSS can fully describe the mechanism of the leakage rate. As the dynamic pressure decreases and the leakage rate increases with the temperature of the medium, the RSS lubrication gradually transitions from MTL to EHL. Thus, the initial medium temperature should be considered to maintain the RSS under the mixed lubrication regime with low friction.

Investigation of lubricant viscosity and third-particle contribution to contact behavior in dry and lubricated three-body contact conditions

The generation of third particles and change in viscosity lead to the gradual degradation of the performance of the machine interface. The generation of third particles may come from wear debris or environmental particles, which form a three-body contact system at the contact interface. The viscosity of the lubricant will also change with the long-term operation of the components. This paper uses a three-body lubrication model to study the influence and interaction of lubricant viscosity change and the presence of third particles on the contact characteristics, including the real contact area, the particle contact area ratio, the solid load percentage, the film thickness, and the evolution of the lubrication regime. The results show that when the interface is in a three-body mixed lubrication regime, the dimensionless total real contact area increases with the increase in particle size and density at the same lubricant viscosity, while the trend is the opposite in dry contact and boundary lubrication interfaces. When viscosity decreases, a three-body contact interface is more prone to entering boundary lubrication than a two-body contact interface, resulting in surface damage. Regardless of surface roughness, particle size, and dry or lubricated contact conditions, the turning point of the contact area (TPCA) phenomenon is usually when the ratio of particle size to surface roughness is 0.8–1.3. Under the same ratio of particle size to surface roughness, the critical load of the TPCA phenomenon increases with the increase in third-particle size and surface roughness, but decreases with the increase in lubricant viscosity and particle density.

The Effect of Ethanol Fuel-Diluted Lubricants on the Friction of Oil Control Ring Conjunction: A Combined Analytical and Experimental Investigation

This paper presents an investigation of the frictional behaviour of three-piece piston oil control rings. A bespoke tribometer replicates the kinematics of the contact between a full oil control ring and the cylinder liner. The three-piece oil control ring is composed of two segments, separated by a waveform-type expander. The experimental results indicate the dominance of a mixed regime of lubrication throughout the stroke. This is particularly the case when the experiments are conducted at 80 °C, a typical engine sump temperature, when compared with those at 20 °C (a typical engine start-up temperature in the UK in the summer). A mixed hydrodynamic analytical model of the oil control ring–cylinder liner tribological interface is employed to apportion frictional contributions with their physical underlying mechanisms. Therefore, combined numerical and experimental investigations are extended to lubricant contamination/dilution by ethanol-based fuels. This study shows that the transition from E10 to E85 would have an insignificant effect on the friction generated in the oil control ring conjunction. This holistic approach, using a detailed predictive l mixed regime of lubrication model and a representative bespoke tribometry, has not hitherto been reported in the open literature.

Tribology behaviour of graphene-modified nanostructured Al2O3/3 % TiO2 coatings under boundary and mixed lubrication conditions

This study is focused on the preparation of a self-lubricating coating via thermal spraying. This coating was designed to control friction and lubrication in both boundary lubrication (BL) and mixed lubrication (ML) regimes. We aimed to compare the tribological behaviour of nanostructured Al2O3/3 % TiO2 and modified nanostructured Al2O3/3 % TiO2 incorporating 3 % GNPs coatings deposited by Oxygen-Fuel (OF). The critical conditions for the transition from BL to ML were experimentally studied using a fundamental tribological concept known as the Stribeck curve. A lubricating film consisting of detached GNPs, Al2O3 nanoparticle debris, and a conventional lubricant effectively covers the surface material and improves the mechanochemical interactions between the surface material and lubricating oil, thereby enhancing the tribological performance. The introduction of GNPs is found to play an effective role in the lubrication regime, leading to an increase in microhardness, a decrease in the friction coefficient and wear volume of the Al2O3/3 % TiO2 coating, and the steel counterpart disc. It noticeably improved the critical load-carrying capacity at different speeds during the transition from BL to ML regimes. The tribo-layer formed on the worn surfaces controls the tribological properties of the Al2O3/3 % TiO2 + 3 % GNPs coatings.

Study on scuffing failure of gas turbine ball bearing based on 3D mixed lubrication and dynamics

Ship gas turbine bearings are subjected to heavy loads and intricate interface micromorphologies, predominantly operate in a mixed lubrication regime. Excessive asperity relative sliding can cause transient high-temperature spikes at contact points, leading to scuffing failures. Dynamic parameters such as bearing friction and clearance, along with vibrations causing fluctuating velocities and loads, further influence the lubrication condition that, in its turn, can increase the risk of scuffing. This study focuses on oil-lubricated gas turbine bearings, accounting for their typical transient working conditions and real surface roughness. The coupling characteristics of ball bearing dynamics, lubrication, and flash temperature are explored. Findings indicate that the most severe contact conditions occur at azimuth angles of 0° and 360°. The risk of ball Scuffing failure exceeds that of the inner raceway. As axial load increases under the azimuth angle of 360°, the area with zero film thickness is increasing, elevating the chances of asperity contact, friction coefficient, and interface temperature, which is detrimental to lubrication. To mitigate pitting and scuffing failures, it's recommended to maintain the bearing's maximum hertz pressure below 1.21 GPa.

Prediction of thrust bearing’s performance in Mixed Lubrication regime

A hydrodynamic thrust bearing could be forced to operate in mixed lubrication regime under various circumstances. At this state, the tribological characteristics of the bearing could be affected significantly and the developed phenomena would have a severe impact on the performance of the mechanism. Until recently, researchers were modeling the hydrodynamic lubrication problem of the thrust bearings either with analytical or with numerical solutions. The analytical solutions are very simple and do not provide enough accuracy in describing the actual problem. To add to that, following only computational methodologies, can lead to time consuming and complex algorithms that need to be repeated every time the operating conditions change, in order to draw safe conclusions. Recent technological advances, especially on the field of computer science, have provided tools that enhance and accelerate the modeling of thrust bearings’ operation. The aim of this study is to examine the application of Artificial Neural Networks as Machine Learning models, that are trained to predict the coefficient of friction for lubricated pad thrust bearings in mixed lubrication regime. The hydrodynamic analysis of the thrust bearing is performed by solving the Average 2-D Reynolds equation numerically. In order to describe the roughness of the profiles, both the flow factors suggested by N. Patir and H.S. Cheng (1978) and the model of J.A. Greenwood and J. H. Tripp (1970) are taken into consideration. Three lubricants, the SAE 0W30, the SAE 10W40 and the SAE 10W60, are tested and compared for a variety of operating velocities and applied coatings. The numerical analysis results are used as training datasets for the machine learning algorithms. Four different ML methods are applied in this investigation: Artificial Neural Networks (ANNs), Multi- Variable Quadratic Polynomial Regression, Quadratic SVM and Regression Trees. The coefficient of determination, R2 is calculated and used to determine the most accurate ML method for the current study. The results showed that ANNs provide very good accuracy in the prediction of friction coefficient compared to the rest of the ML models discussed.

The friction of surface textured cylinder liner segments modified by Direct Laser Writing and Direct Laser Interference Patterning processes

Direct Laser Writing (DLW) using ultrashort pulsed lasers and galvanometer scanning, has become an established process for surface texturing, allowing an extensive variety of feature geometries and texture designs to be produced. The minimum feature sizes that can be generated are however limited by factors such as the laser output power, beam diameter, beam wavelength and associated optics. To address the need to produce even smaller texture features, Direct Laser Interference Patterning (DLIP) has been developed, which utilises bespoke beam splitting and converging optics to produce periodic interference fringing and enabling surface textures in the micrometric size range to be produced.In this study, the texturing of cast iron cylinder liner segments using DLW and DLIP processes is explored, to assess their tribological performance in high efficiency internal combustion engines, such as for hybrid applications. To facilitate the designs of textures and to capture high resolution data on surface interactions at specific points in the combustion cycle, the segmentation of cylinder lines and piston rings is undertaken. The results of tribology tests covering combinations of high and low speeds and contact pressures, have shown that the mechanical honing of cylinder liners could potentially be replaced with the cleaner DLIP texturing as an alternative.Optimised texture designs using the DLW process achieved up to a 54 % reduction in kinematic friction between the piston ring and textured cylinder liner surfaces, compared with mechanically honed cylinder liner segments. These results have indicated that DLW texturing offers the best improvement in friction performance in the boundary and mixed lubrication regimes at higher contact pressures, at both lower and higher sliding speeds. The results also provide useful comparative data on the performance of meso and micrometric scale textures and how they can be best applied to cylinder liners.

Prediction of mechanical efficiency of spur gear pairs based on tribo-dynamics model of multi-tooth meshing

The prediction of load-dependent losses of gear pairs is a topic of constant concern, which depends largely on the modelling of friction coefficient in tooth meshing. Although the current load-sharing based models can handle the friction coefficient in mixed elastohydrodynamic lubrication regimes, dynamics behaviour of each tooth pair or the time variation of direction of normal contact load is not taken into consideration. This study proposes a new method for modelling efficiency of spur gear pairs in transient operating conditions by coupling the multi-tooth meshing model with the friction coefficient model. The friction coefficient is modelled through load-sharing function, and the gear dynamics model is established with regard to alternate meshing of single-double tooth pair and time variation of direction of normal load and mesh stiffness. The model is validated by comparing the calculated results of friction coefficients and load-dependent losses with experimental data in published literatures, showing good agreement. At the end, the efficiency model is applied to two types of gear pairs to investigate the influence of tooth shapes and operating parameters on the load-dependent losses of gear pairs.

Effects of surface morphology on the performance of water-lubricated thrust bearings

Purpose As the crucial support component of the propeller power system, the reliability of the operation of submersible pumps is influenced by the lubrication performance of water-lubricated thrust bearings. When the water-lubricated thrust bearings are under start-stop or heavy load conditions, the effect of surface morphology is crucial as the mixed lubrication regime is encountered. This paper aims to develop one mixed lubrication model for the water-lubricated thrust bearings to predict the effects of surface skewness, kurtosis and roughness orientation on the loading carrying capacity and tribological behavior. Design/methodology/approach This paper developed one improved mixed lubrication model specifically for the water-lubricated thrust bearing system. In this model, the hydrodynamic model was improved by using the height of the rough surface and its probability density function, combined with the average flow model. The asperity contact model was improved by using the equation for the Pearson system of frequency curves to characterize the non-Gaussian aspect of surface roughness distribution. Findings According to the results, negative skewness, large kurtosis and lateral surface pattern can improve the tribological performance of water-lubricated thrust bearings. Optimizing the surface morphology is a reasonable design method that can improve the performance of water-lubricated thrust bearings. Originality/value In this paper, one mixed lubrication model specifically for the water-lubricated thrust bearing with the effect of surface roughness into consideration was developed. Based on the developed model, the effect of surface morphology on tribological behavior can be evaluated. Peer review The peer review history for this article is available at: https://publons.com/publon/10.1108/ILT-08-2023-0247/

The Enhancement of Oil Delivery and Bearing Performance via a Guiding-Structured Nozzle under Oil–Air Lubrication

The oil–air lubrication method is specifically employed for high or ultra-high-speed spindle rolling bearings. Under high-speed conditions, the air curtain formed inside the bearing cavity obstructs oil delivery, thereby limiting further increases in spindle rotation speed. To enhance oil delivery capability, a guiding-structured nozzle has been developed to concentrate the jet flow and improve penetration through the air curtain. Tests were conducted on an oil–air lubricated bearing test bench to investigate the impact of nozzle structures and oil types on torque and temperature rise. The results demonstrate that compared to conventional nozzles, the guiding-structured nozzle requires smaller optimal amounts of oil supply, indicating its superior ability to deliver oil. Further examination of oil jet patterns and droplet distributions confirms that the guiding-structured nozzle provides a more concentrated jet flow with uniform distribution and smaller droplet sizes in diameter. These characteristics contribute to highly efficient oil delivery. Additionally, synthetic oils reduce droplet size, torque, and temperature rise in mixed lubrication regimes due to their formation of an anti-friction absorption layer on rubbing surfaces.

From mixed to hydrodynamic regime in lubricated sliding of carbon fiber tows

Carbon fiber tows are used as reinforcement material in composite elements. To ensure mechanical integrity of the composite structure, it is important to ensure a good positioning of the carbon fibers in the mold during the manufacturing process. The positioning of the carbon fiber tows depends greatly on the friction with the mold. In the present paper, the friction between a carbon tow placed on a cylindrical pin rubbing against a rotating glass disk is experimentally studied in both dry and resin-lubricated conditions on a dedicated tribometer. The tests are performed for different loading levels and sliding speeds. The results are compared to numerical simulations taking into account contact mechanics and lubrication. The tows are modeled using a simple elastic foundation approach coupled with the Reynolds equation when the resin flow is considered. It appears that the impact of the deformability of the tow on its frictional behavior is significant. It conditions the transition between mixed and hydrodynamic lubrication regimes.