International Colloquium Tribology
ict
expert verlag Tübingen
125
2022
231
An innovative multi-scale approach to reduce friction of automotive lubricants from hydrodynamics to boundary lubrication
125
2022
Denis Mazuyer
Juliette Cayer-Barrioz
ict2310045
23rd International Colloquium Tribology - January 2022 45 An innovative multi-scale approach to reduce friction of automotive lubricants from hydrodynamics to boundary lubrication Prof. Denis Mazuyer École Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Surfaces, CNRS UMR5513, Écully, France Corresponding author: denis.mazuyer@ec-lyon.fr Juliette Cayer-Barrioz École Centrale de Lyon, Laboratoire de Tribologie et Dynamique des Surfaces, CNRS UMR5513, Écully, France 1. Introduction The tribological mechanisms, associated in particular with the formation, the maintenance of a lubricating film with fluids of low viscosity and its capacity to circumscribe the dissipation by friction to a value as low as possible were investigated over all the lubrication regimes. The authors tackled the problem by treating separately the compliant contacts operating at low pressure (crankshaft bearing) and the non-compliant contacts subjected to high pressures (timing, piston ring/ liner). An experimental methodology was implemented covering 11 decades of sliding velocities and contact pressure, from 10-10 to 10 m/ s and up to few GPa, thanks to the development of IMOTEP research platform. As part of this study, the in situ multi-scale tribological analysis allowed us, among other things: • to study the effects of a progressive confinement of the contact and to evaluate the resistance to interfacial shear of the formed films; • to identify the mechanisms at the origin of the generation of lubricating films for different tribological conditions, representative of the operating conditions of the upper engine contacts; • to establish generalized Stribeck curves and friction / pressure / velocity maps. Figure 1: Multi-scale analysis of the lubrication regimes 2. Experimental strategy All the lubrication regimes were analyzed for various lubricants and surfaces using three dedicated tribometers (see figure 1) developed at the Ecole Centrale de Lyon: The boundary regime (BL) was simulated using a molecular tribometer (ATLAS [1]) making it possible to measure the tribological behavior of adsorbed molecular layers, under moderate contact pressure (from 1 MPa to 50 MPa), as a function of the sliding speed (from 0.1 nm / s to 100 nm / s); The mixed regime (ML), the elastohydrodynamic regime (EHL) and the transitions between these lubrication regimes were investigated thanks to the IRIS tribometer [2] that allows independent and simultaneous control of the sliding and entrainment speeds to achieve Stribeck and traction curves. Positioned under an infrared spectrometer, the chemical species passing through the contact can also be analyzed; The hydrodynamic regime (HL) was studied on a fully-instrumented journal bearing bench (shaft diameter = 5 mm, radial clearance = 5 µm) in which the position of the shaft, the friction torque, the oil temperature and the lubricating film rupture zone are measured as a function of time over a speed range of up to 10,000 rpm [3]. The confinement ratio of the lubricant film has an order of magnitude of 1 to 1000, in these three test rigs. To illustrate this strategy, this talk was focused on the tribological behaviour of a Group III+ base oil (BO) and its mixture with a PMA Viscosity Index Improver (BO + PMA). 3. Main results 3.1 Lubricant film thickness For a PMA solution concentrated at 3%w/ w, boundary film thickness was measured at about 10 nm on metallic surface, using both the ATLAS molecular tribometer and IRIS tribometer (see Figure 2). This boundary film was mechanically characterized: it is highly elastic, its shear 46 23rd International Colloquium Tribology - January 2022 An innovative multi-scale approach to reduce friction of automotive lubricants from hydrodynamics to boundary lubrication elastic modulus G is equal to 90 MPa under a 50 MPa contact pressure, and this film is purely repulsive. An additional image analysis of the interferograms grabbed with IRIS tribometer showed that patches of PMA covered all the Hertzian contact in BL. This boundary film thickness was detected at low entrainment velocity (less than 20 mm/ s). At higher velocity, in EHL regime, the film thickness followed the classical elastohydrodynamic prediction as shown in figure 2. In order to better understand the in situ and in real-time lubricating film nature, and to identify the different chemical species entrained in and around the contact, an infra-red spectroscopy analysis was performed: the analysis of the response of the C=O bond allows us to quantify the local concentration in polymer. In hydrodynamic regime, the 5mm hydrodynamic film was established, showing the major influence of the fluid rheology. Figure 2: Evolution of the lubricant film thickness for BO and the mixture BO + PMA versus the entrainment product. 3.2 Friction dissipation The friction of this PMA solution was analyzed at low velocity, when the two boundary films were in contact: high friction values were measured, associated to a viscoelastic contribution. However, the existence of these PMA boundary films induced a reduction in friction compared to the base oil. At higher velocity and higher pressure, a Stribeck curve was measured. We were able to show that the existence of the PMA boundary film shifted the mixed/ EHL regime transition towards lower velocities compared to that measured with the base oil. The shear was also localized in the hydrodynamic film. In hydrodynamic regime, for few mm thin films, two regimes were identified: a non-thermal regime of fluid shear, for acceleration and deceleration, and a thermal regime during which the temperature increases due to shearing. Apart from changing the viscosity-temperature relation, the presence of PMA due to adsorption also modified the cavitation zone extension. The non-Newtonian properties of the fluid under high shear were also investigated. Figure 3: Evolution of the viscous shear stress versus the entrainment product at the contact temperature for BO and the mixture BO + PMA. 4. Conclusion This innovative multi-scale approach allows one to identify lubrication mechanisms, in terms of film thickness and friction, over 11 decades of sliding velocities and contact pressure, thanks to the development of IMO- TEP research platform. In addition, an in situ analysis is developed based on a coupled optical measurement, infra-red and mechanical spectroscopy. On an illustrative example, we showed here that the interactions between the metallic surface and a viscosity-improver additive induced the formation of a boundary film that protected the surfaces, shifted the lubrication regime transitions to lower velocities and reduced the friction mechanisms in all regimes, via different mechanisms including shear localization, cavitation extension, etc. References [1] Crespo, A., Mazuyer, D., Morgado, N., Tonck, A., Georges, J.-M., and Cayer-Barrioz, J., “Methodology to Characterize Rheology, Surface Forces and Friction of Confined Liquids at the Molecular Scale Using the ATLAS Apparatus”, Trib Lett. 65, 4, 2017, Article number 138. [2] Bonaventure, J., Cayer-Barrioz, J., and Mazuyer, D., “Transition Between Mixed Lubrication and Elastohydrodynamic Lubrication with Randomly Rough Surfaces”, Trib Lett. 64, 3, 2016, Article number 44. [3] Barazzutti, J., Cayer-Barrioz, J., and Mazuyer, D., “A new in-situ methodology for understanding hydrodynamics journal bearing”, 8th International Tribology Conference, September 17 - 21, 2019, Sendai.
