Publications HAL de la collection SKFCHAIR

2023

titre
A Quantitative Approach to EHD Friction Prediction Based on Rheometry and Molecular Dynamics Simulations
auteur
Ruibin Xu
article
Engineering Sciences [physics]. Insa Lyon, 2023. English. ⟨NNT : ⟩
resume
This thesis presents a quantitative approach to elastohydrodynamic (EHD) friction prediction, based on a combination of experimental rheometry data and molecular dynamics (MD) simulations. The approach is applied to two fluids of different natures: a lubricant squalane (SQ) and a traction-like fluid benzyl benzoate (BB). The Newtonian viscosity of the fluids is determined using measurements from high-pressure viscometers (HPV) taken from the literature, and a novel Newtonian viscosity model is proposed in this work, which is based on an existing thermal scaling model in the literature. Subsequently, a comprehensive Eyring stress law, covering a wide range of temperature and pressure conditions, is constructed from non-equilibrium molecular dynamics (NEMD) simulations. The obtained Newtonian viscosity and Eyring stress are used to build Eyring-type generalised viscosity models for both fluids. These are implemented into a finite element (FE) model of a lubricated contact in the elastohydrodynamic regime, taking into account non-Newtonian and thermal effects (TEHLnN) for friction prediction. The results are compared with friction measurements performed in a tribometer under the same contact conditions and show good agreement. Notably, the friction plateau and the thermal thinning regime observed experimentally are accurately reproduced by the TEHLnN model. Further research was carried out to investigate the origin of friction plateaus. The results suggest that friction plateaus result from a combination of non-Newtonian effects and thermal effects. The work also reveals that the thermal effect arises almost simultaneously with the non-Newtonian effect. This work presents a crucial step towards “true” friction prediction and quantitative EHL, bringing together experimental rheometry, molecular dynamics simulations, and contact modelling.
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2022

titre
Elastohydrodynamic lubrication with mixtures of oil and operating fluids: an industrial challenge
auteur
Fan Zhang
article
Fluids mechanics [physics.class-ph]. Institut National des Sciences Appliquées de Lyon, 2022. English. ⟨NNT : ⟩
resume
The fluid that separates the contacting elements in many industrial applications is often a mixture of oil and operating fluids. This work aims to provide a better understanding of lubrication with mixtures in elastohydrodynamic contacts in order to anticipate appropriate lubrication system performance. The oil/refrigerant mixture and the oil/water mixture, representing respectively miscible fluids and immiscible fluids, are thoroughly investigated in this study. The essential questions differ depending on the characteristics of the mixtures. The main issue for oil/refrigerant solution is at the contact outlet, where the refrigerant may evaporate. An original cavitation modeling with thermal effects for oil/refrigerant solutions is reported in this work. A comparison with other cavitation models from the literature is made, highlighting the necessity of considering the refrigerant solubility in oil for such problems. Indeed oil/refrigerant mixtures may dramatically reduce the amount of liquid oil for the next contact compared to the case of pure oil cavitation. Besides this, the problem of water droplets in oil emulsion is addressed at the contact inlet to evaluate the risks of water droplet contamination. A micro-sized water-in-oil emulsion flow is directly observed at the EHL point contact inlet. In parallel, a numerical approach was developed to investigate the impacts of various operating parameters on the flow of water-in-oil emulsion. The results of this work give a possible explanation for the contradictory findings in the literature and reveal the critical conditions under which water-in-oil emulsion would influence the EHL contacts.
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https://hal.science/tel-04492782/file/Thesis_correction_FZ.pdf BibTex

2020

titre
Lubricant starvation in elastohydrodynamic large-size spinning contacts
auteur
Alberto Porras Vazquez
article
Mechanics [physics.med-ph]. Université de Lyon, 2020. English. ⟨NNT : 2020LYSEI109⟩
resume
This thesis is dedicated to the study of large-size spinning contacts located at the interface between the roller-end and the flange in rolling-element bearings. The main goal of the study is to evaluate the influence of lubricant starvation on the film thickness distribution of the contact and analyze how spinning might affect this mechanism. Due to its importance in the reliability and performance of the bearing, the focus is set of the local minimum film thickness found at the low velocity region of the contact area. To tackle this problem, a dual numerical-experimental approach is proposed. The film thickness distribution of spinning contacts is investigated numerically by means of a finite element model previously validated by two dedicated test rigs: Jerotrib and Tribogyr. The simulation of different operating, kinematic, geometric and lubrication conditions enables to write an analytic expression for predicting the aforementioned critical film thickness. At the same time, novel techniques to experimentally induce and control starvation in the contact are implemented into both test rigs and their results are contrasted with those of the simulation. It is demonstrated that the effects of spinning and starvation add up, so that the film thickness distribution of the spinning contact remains asymmetric but tends to a more Hertzian, and therefore thinner, distribution when limiting the oil supply upstream of the contact’s inlet.
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https://theses.hal.science/tel-03187008/file/these.pdf BibTex
titre
Non-intrusive CdSe-based quantum dots for sensing pressure and temperature in lubricated contacts
auteur
Tarek Seoudi
article
Mechanical engineering [physics.class-ph]. Université de Lyon, 2020. English. ⟨NNT : 2020LYSEI009⟩
resume
This thesis is dedicated to the measurement of local pressure and temperature and to compare the heat generation in all-steel and silicon nitride-steel (hybrid) elastohydrodynamic (EHD) contacts. The ultimate goal of this work is to develop a new non-intrusive in situ technique, exploiting the sensitivity of the photoluminescence (PL) of CdSe/CdS/ZnS quantum dots (QDs) to pressure and temperature. Dispersible in small concentration in lubricants, it is shown that the QDs doesn’t modify the rheological behavior of the carrier fluid and that shearing is not perturbative to the QDs PL response. The calibration of QDs in the suspension confirms the QDs PL dependence on temperature and pressure. The in situ measurements were conducted in EHD contacts using a ball-on-disc test rig. Comparisons between pressure and temperature measurements and predictions, using an in–house finite element thermal EHD model, showed a good agreement which demonstrates the feasibility of the proposed methodology. The effects of sliding and normal loading on pressure, temperature and heat generation are indicated. The effect of the thermal properties of the solid materials is underlined and the partition of the generated heat between the contacting solids is investigated. The energy equilibrium between the mechanical energy and the internal thermal energy generated by compression and shearing is demonstrated by comparing experimental power losses and numerical heat generation, in steel-steel and hybrid contacts.
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https://theses.hal.science/tel-02900672/file/these.pdf BibTex
titre
Film thickness build-up in highly loaded lubricated contacts under Zero Entrainment Velocity condition
auteur
Bilel Meziane
article
Mechanical engineering [physics.class-ph]. Université de Lyon, 2020. English. ⟨NNT : 2020LYSEI005⟩
resume
Highly loaded lubricated contacts are often studied in rolling/sliding conditions. In those cases, the entrainment of lubricant in a so-called “oil wedge” explains the existence of a separating film thickness. However, in a number of industrial applications, the contact is subjected to opposite surface velocities. In such cases, there is a Zero Entrainment Velocity (defined as the average velocity of the two surfaces) of the fluid. The film thickness prediction formulae developed in the literature for rolling contacts are unusable. In this thesis, the physical phenomena leading to a film build-up under Zero Entrainment Velocity condition are elucidated. A finite element model is used in order to facilitate in-situ measurements. It aims to describe the behaviour of the contact in thermal and transient conditions. In the stationary regime, the numerical values are compared with a very good agreement to a set of results obtained via a tailored experimental campaign. This dual approach enables a quantitative description of the influence of the contact load, surface velocities and external temperature on the film thickness under ZEV condition. Then, the relative influence of the thermal and squeeze effects is studied. Depending on the ratio between the characteristic loading time and the characteristic thermal time, these two effects can show a beneficial synergy for the contact.
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https://theses.hal.science/tel-02882664/file/these.pdf BibTex

2019

titre
A molecular approach to the ultimate friction response of confined fluids
auteur
Alejandro Porras Vazquez
article
Fluids mechanics [physics.class-ph]. Université de Lyon, 2019. English. ⟨NNT : 2019LYSEI087⟩
resume
In order to control energy losses in mechanical systems, a thin film of lubricant is often introduced between the solids in contact. The lubricated point contacts operate in the elastohydrodynamic regime, characterized by high pressures (of the order of GPa) and thin film thicknesses (of the order of 100 nanometers). At high shear rates, the fluid may exhibit a limiting shear stress whose physical origin is still uncertain. At present, the empirical models available for the prediction of friction fail to describe the ultimate response of lubricants at these severe operating conditions. In addition, in-situ experimental analysis is very difficult to achieve due to confinement and high pressures. Thus, in this thesis, the problem is approached from the angle of modeling at the atomic scale. The shear behavior of three fluids (a traction fluid, a model lubricant and an industrial lubricant for the aerospace industry) is analyzed by Molecular Dynamics Simulation. The numerical results are then compared qualitatively and quantitatively with experimental tests. The friction response is independent of the velocity profile in the confinement thickness, the latter appearing rather as a consequence of boundary conditions at the surfaces. The limiting friction regime naturally occurs when the lubricant is subjected to thermodynamic conditions characteristic of a solid state. In this case, the dynamics of the molecules is strongly slowed down. The activation energy increases rapidly with the pressure, so that the diffusion becomes negligible at high pressure, even at the severe shear rates imposed in the Molecular Dynamics simulations. The macroscopic response to this phenomenon is thus a saturation of the value of friction. This work ends by laying the foundations of a modeling that will allow the prediction of lubricated friction under severe conditions.
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https://theses.hal.science/tel-02882466/file/these.pdf BibTex

2017

titre
Ultimate behavior of confined fluids under very high pressure and shear stress
auteur
Serigne Ndiaga Ndiaye
article
Mechanics [physics.med-ph]. Université de Lyon, 2017. English. ⟨NNT : 2017LYSEI104⟩
resume
Friction in highly loaded lubricated contacts is a complex issue. Indeed, it highly depends on the lubricant rheological behaviour in the Hertzian region, which is not well known under such high pressure and high shear stress. Various experimental methods have been developed to explain the plateau-like behaviour in friction curves referred to as the limiting shear stress (LSS), but none of them provided a full picture of the real mechanisms involved. In a continuation of these efforts, some investigations are presented in this manuscript. The first challenge in this work is to carry out friction measurements under nominal isothermal conditions, meaning that even if thermal effects must occur in any friction measurement, it is possible to minimize them and to make the results almost insensitive to a weak energy dissipation within the experimental volume of interest. Minimizing shear heating of the lubricant help us to focus on the mechanical origin of the LSS and to better characterize its dependence to pressure and temperature. That’s why, first of all, a series of experiments was performed on two lubricants, a pure diester fluid (benzyl benzoate), and a commercial turbine mineral oil (Shell T9) with varying entrainment velocities. This allow us first to directly observe the influence of the lubricant shear heating on the LSS values and then to determine the experimental conditions which limit this thermal effect while ensuring a full film regime. The second objective is to characterize the frictional behavior of both lubricants under nominal isothermal conditions and over a wide range of pressure (up to 3 GPa) and temperature (up to 80°C) in order to establish a new uncoupled model to describe the temperature and pressure dependence of the limiting shear stress under highly loaded conditions. Finally, the study focuses on the understanding of the microscopic behavior of lubricants under extreme shear and pressure conditions. In situ Raman and Brillouin spectroscopy investigations were also conducted under static conditions, in order to study the lubricant phase changes under various pressure and temperature conditions.
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https://theses.hal.science/tel-02061350/file/these.pdf BibTex

2016

titre
Non-elliptical point contacts : The Torus-on-Plane conjunction
auteur
Jean-David Wheeler
article
Mechanics [physics.med-ph]. Université de Lyon, 2016. English. ⟨NNT : 2016LYSEI131⟩
resume
This thesis is dedicated to the study of torus on plane contacts under various operating conditions. They can be found at the interface between the torus roller-end and the flange in roller bearings. The first challenge of this thesis is to deal with unusual mating geometries. The other challenge is the presence of a complex kinematic which operates in these contacts. In order to further develop the understanding of such a contact, a dual approach (experimental and numerical) is adopted. The Jérotrib test-rig enables a first study, by considering that the élastohydrodynamic torus on plane contact can be modelled by an elliptical equivalent contact. Thanks to a differential colorimetric interferometry method which was improved and adapted during the thesis, precise film thickness measurements are carried out under a rather wide range of operating conditions. A thermo-elastohydrodynamic numerical model is developed and validated by comparing its results to the ones of the test-rig. A numerical study on film forming is then proposed and the role of the contact ellipticity is investigated. The numerical model is improved in order to take into account the actual shape of the solids. A film thickness validation of the model is proposed, thanks to measurements performed on the Tribogyr test-rig. The operating conditions are very similar to the one encountered in actual bearings, and the mating solids have representative geometries: it is an actual torus-on-plane contact. It is demonstrated that the lubricant shearing is responsible for the solids temperature rise, which in its turn, reduces the film thickness. It appears mandatory to be able to predict this global warming of the bodies. It is also demonstrated that the pressure and film thickness distributions lose their symmetry because of the spinning kinematic and the solids shape. However, the behaviour of the torus-on-plane contact appears very similar to the one of an elliptical equivalent contact, apart from some limit cases.
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https://theses.hal.science/tel-01791538/file/these.pdf BibTex
titre
Molecular simulation of an ionic liquid as lubricant: from bulk rheology to nanoconfinement
auteur
Nicolas Voeltzel
article
Mechanical engineering [physics.class-ph]. Institut national des Sciences Appliquées Lyon, 2016. English. ⟨NNT : ⟩
resume
Increasing economic and environmental constraints tend to reduce the amount of lubricant used in mechanical systems. This results in nanometric film thicknesses in the contact areas, leaving only a few layers of lubricant molecules to ensure the separation of the surfaces. To meet this challenge, new fluids are being considered such as ionic liquids which feature a great potential as lubricants. Through Molecular Dynamics simulations, the rheological response of an ionic liquid to different conditions of temperature, shear and pressure is first characterized in detail. The ability of the ionic liquid to lubricate thin films is confirmed. Besides, this study goes back over the classic analytical models used in rheology. The ionic liquid is then nanoconfined between two representative surfaces of a steel-steel contact. The combined effects of wall slip, shear-thinning and temperature rise provide answers to explain the saturation of the shear stress. Finally, the influence of different low friction surface coatings is analyzed. According to the material used, very different dynamical behaviors occur. Moreover, it is shown that the use of polar surfaces significantly impacts the response of ionic liquid lubricated contacts.
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https://hal.science/tel-02172562/file/Manuscrit_VersionFinale.pdf BibTex
titre
Particle entrapment in EHD contacts - Aerospace applications
auteur
Vincent Strubel
article
Structural mechanics [physics.class-ph]. Université de Lyon, 2016. English. ⟨NNT : 2016LYSEI098⟩
resume
Contact lubrication is essential in a wide range of mechanical systems like rolling element bearings (REBs). A minimum quantity of clean lubricant all along the bearing life is necessary but difficult to ensure. In fact, lubricants contain inevitably wear debris or external particles, like dust. Carried by the lubricant in the vicinity of elastohydrodynamic (EHD) contacts, particles can be entrapped with disastrous consequences for contacting surfaces. Entrapment of micrometric particles in submicrometric contacting gaps means irreversible damages for the surfaces. Damages weaken the surfaces and reduce significantly the REBs lifetime. The goal of this work is to analyze the critical particle entrainments in the contact inlet. Entrapment of steel spherical particles was investigated from the numerical and experimental point of view. Firstly, the phenomenology of entrapment was explored with a new experimental method based on Particle Image Velocimetry (PIV) technique installed on a tribometer. It enabled the evaluation of velocity profiles in the contact inlet and the tracking of particles within EHD contacts. Secondly, a numerical modelling of the inlet flow for EHD contacts, including the particle tracking, was developed. Finally, tests on a twin-disc machine with a controlled level of well-defined contamination were conducted to validate previous conclusions. A first set of results showed that particle entrapment is highly dependent on the lubricant velocity profile. Depending on contact geometry, from point to wide elliptical contacts, different entrapment probability were revealed. Surprisingly, increasing contact width with wide elliptical contacts leads to a drop of entrapped particles. It was demonstrated that this phenomenon is due to backflows occurring upstream from these contacts. Introducing a hybrid pair of contacting materials (silicon nitride–steel), dents on the surfaces due to entrapped particles were explored. It has been confirmed that silicon nitride surface offers a real ability to resist to indentation. It was also noticed that the entrapment probability for silicon nitride–steel contacts is equivalent to a steel–steel one.
Accès au texte intégral et bibtex
https://theses.hal.science/tel-01694082/file/these.pdf BibTex