David Philippon

A Dual Experimental/Numerical Approach for Film Thickness Analysis in TEHL Spinning Skewing Circular Contacts

Studies on the behaviour of rolling bearings show that the position as well as the loading of the rolling element at the flange roller-end contact depends on the momentum created on the roller track caused by local friction variability. This situation is characterized by a skew angle that varies within a limited range in the application influenced also by the clearance in the cage pockets. This paper provides some elements which contribute to increase the understanding of the physical phenomena occurring in lubricated spinning contacts under skew. An experimental investigation is carried out using a unique in-house test rig called Tribogyr, dedicated to large size-contacts with spinning and skewing kinematics. Alongside the experiments, a numerical analysis is conducted, based on a finite element approach, aiming to take into account the multiphysics aspect of this thermo-elastohydrodynamic problem. The model and the experiments show a good agreement. The skew effects on film thickness of spinning flange/roller-end contacts are characterized by a global decrease of central film thickness, mainly due to high shear of the lubricant, involving thermal thinning. The shallow dimple captured by the in situ measurements of film thickness can be explained as a consequence of the viscosity wedge due to different thermal conductivities between glass and steel. The dual experimental–theoretical approach was a useful method that help to study separate phenomena which are naturally coupled each other.

Trimethyl-phosphite dissociative adsorption on iron by combined first-principle calculations and XPS experiments

The reaction of trimethyl-phosphite, TMPi, with a clean Fe(110) surface has been investigated by ab initio calculations. The most stable configurations and energies are identified for both molecular and dissociative adsorption. The calculated reaction energies indicate that dissociation is energetically more favorable than molecular adsorption and we provide a description of the dissociation path and the associated energy barrier. In situ XPS analysis of adsorbed TMPi on metallic iron confirmed molecular chemisorption and dissociation at high temperature. These results shed light on the mechanism of phosphorus release from organophosphites at the iron surface, which is important for the functionality of these phosphorus-based additives, included in lubricants for automotive applications.

On the crucial role of ellipticity on elastohydrodynamic film thickness and friction

The study reported here deals with elastohydrodynamic point contacts and it is focused on the influence of contact ellipticity. In five velocity–load reference cases, ellipticity was varied from slender to wide configurations, including the circular contact. For each case, Hertzian pressure, Hertzian area, load, and entrainment velocity were kept constant while the ellipticity was varied by changing the curvature radii. In this context, the maximum central film thickness did not occur for the infinitely wide contact, but for a slender configuration close to the circular case. Moreover, the minimum film thickness reached its optimum for a wide but finite elliptical contact. For low ellipticity ratios, specific film thickness features were obtained. In particular, very high central/minimum film thickness ratios are found. The cause of these behaviors was found in the change of the convergent shape. When the ellipticity was varied, the Poiseuille flows parallel and transverse to the entrainment direction were significantly modified and these modifications were quantitatively analyzed for the different cases. The competition between the Couette and the Poiseuille flows was totally different between the narrow and the wide elliptical contact, and this change was responsible for the film thickness variations with ellipticity. Ellipticity also had an effect on friction as it influenced the maximum pressure which in turn impacts the fluid viscosity.

Rheological Considerations on Polymer-Based Engine Lubricants: Viscosity Index Improvers versus Thickeners—Generalized Newtonian Models

ABSTRACT The development of high-performance lubricants to decrease engine friction and then reduce fuel consumption remains a major challenge for oil manufacturers. Viscosity index improvers (VIIs) are additives used for decades to reduce the dependency of the lubricants viscosity on temperature to maintain an acceptable lubrication in harsh conditions. Distinction between VIIs and thickeners in realistic engine conditions is of primary interest for oil manufacturers in order to optimize the formulation process. In this context, rheological studies can provide clear insights into the actual effect of such polymeric additives. The behavior of a simplified automotive lubricant is investigated at different temperatures and high pressure and high shear stress and modeling of the properties is proposed. Various polymers of different molecular weights and conformations were mixed in a hydrocracked mineral base oil. The viscosity variations with temperature, pressure, and shear stress, obtained from specific rheometers, were then represented by different models, namely, a Vogel-Fulcher-Tamman expression, a modified Williams-Landel-Ferry correlation, and a Carreau-Yasuda equation. Different rheological responses were observed and allowed the distinction between VIIs and thickeners under different temperature and pressure conditions. The hydrodynamic radii analysis provided an explanation of the rheological responses but also highlighted the limits of the Carreau-Yasuda equation when applied to VIIs. Therefore, the Zhang expression, based on the Maxwell model and applied here to low viscoelastic solutions, is proposed as an appropriate alternative.

Quantum dots to probe temperature and pressure in highly confined liquids

A new in situ technique for temperature and pressure measurement within dynamic thin-film flows of liquids is presented. The technique is based on the fluorescence emission sensitivity of CdSe/CdS/ZnS quantum dots to temperature and pressure variations. In this respect, the quantum dots were dispersed in squalane, and their emission energy dependence on temperature and pressure was calibrated under static conditions. Temperature calibration was established between 295 K and 393 K showing a temperature sensitivity of 0.32 meV K−1. Pressure calibration was, in turn, conducted up to 1.1 GPa using a diamond anvil cell, yielding a pressure sensitivity of 33.2 meV GPa−1. The potential of CdSe/CdS/ZnS quantum dots as sensors to probe temperature and pressure was proven by applying the in situ technique to thin films of liquids undergoing dynamic conditions. Namely, temperature rises have been measured in liquid films subjected to shear heating between two parallel plates in an optical rheometer. In addition, pressure rises have been measured in a lubricated point contact under pure rolling and isothermal conditions. In both cases, the measured values have been successfully compared with theoretical or numerical predictions. These comparisons allowed the validation of the new in situ technique and demonstrated the potential of the quantum dots for further mapping application in more complex and/or severe conditions.

A Generalised Differential Colorimetric Interferometry Method: Extension to the Film Thickness Measurement of Any Point Contact Geometry

ABSTRACT Whereas industrial elastohydrodynamic (EHD) contacts are generally noncircular, most experimental observations are made on sphere-on-plane conjunctions. The circular case is indeed a specific elliptical case, and it was widely investigated. The differential colorimetric interferometry (DCI) technique was often used to perform precise film thickness measurements in circular EHD contacts. From a single picture of the dynamic contact, it enables mapping the film thickness of the full conjunction, and postprocessing can be applied afterwards. Moreover, it is possible to record sequences at relatively high-frequency sampling. However, until now, the method could not be directly applied to noncircular conjunctions. In the present article, a generalized DCI method is proposed and assessed by several static and EHD validation cases for elliptical and torus-on-plane contact geometries. This new method no longer necessitates particular requirements on the contact shape while retaining the advantages of the original DCI method. It allows precise film thickness measurements in realistic industrial EHD contacts and opens the way for new experimental observations.

Tribochemical Competition within a MoS2/Ti Dry Lubricated Macroscale Contact in Ultrahigh Vacuum: A Time-of-Flight Secondary Ion Mass Spectrometry Investigation

Controlling and predicting the tribological behavior of dry lubricants is a necessity to ensure low friction, long life, and low particle generation. Understanding the tribochemistry of the materials as a function of the environment is of primary interest as synergistic effects exist between the mechanics, the physicochemistry, and the thermodynamics within a contact. However, in most studies the role of the coating internal contaminants in the process is often discarded to the benefit of a more common approach in which the performances of the materials are compared as a function of different atmospheric pressure environments. The study focuses on the understanding of the tribochemical processes occurring between the materials and their internal contaminants inside an AISI440C contact lubricated by a MoS2/Ti coating. Time-of-flight secondary ion mass spectrometry is used to study at the molecular level, the material before and after friction. Friction tests with different durations are performed in ultrahigh vacuum at the macroscale to stay relevant to the real application (space). The adsorption/desorption of gaseous species during friction is monitored by mass spectrometry to ensure reliable study of the tribochemical processes inside the contact. The study shows that a competition exists between the Ti- and MoS2-based materials to create the appropriate lubricating materials via (i) recrystallization of MoS2 materials with creation of a MoS xO y material via reactions with internal contaminants (presumably H2O), (ii) reaction of Ti-based materials with internal contaminants (mostly H2O and N2). The biphasic material created is highly similar to the one created in both humid air and dry N2 environments and providing low friction and low particle generation. However, the process is incomplete. The study thus brings insight into the possibility of controlling friction via a rational inclusion of reactants in a form of contaminants to control the tribochemical processes governing the low friction and long life.