Aus Wissenschaft und Forschung / TAE-Plenarvorträge 33 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 1 Introduction The development of high-performance lubricants to insure the separation of mechanical parts, to reduce engine friction and fuel consumption remains a major challenge for oil manufacturers. Polymer additives called Viscosity Index Improvers (VI improvers) have been used for decades to limit the dependency of the lubricant’s viscosity on temperature, to maintain an acceptable hydrodynamic lubrication at high temperature, without experiencing excessive frictional and thermal losses at low temperature. This work focuses on understanding the role of VI improvers in engine lubricants. It aims to bridge their tribological response with their rheological behavior. The objective here is to quantify and clarify the effect of polymer addition on the lubricants performance in lubricated conditions, more especially in the elastohydrodynamic (EHD) and very thin film (VTF) regimes. 2 Experimental details Simplified lubricants are studied, composed of polymers of different chemistry, molecular weight and structure (PAMA-C, PAMA-L, PISH-S, OCP-LE). They were added to a unique base oil (group III) in such a way to comply with a HTHS value of 2.6 mPa.s (10 +6 s -1 at 150 °C). This led to different polymer concentrations, around 1.2 % w/ w for PISH-S and OCP-LE, and of 3.4-4.4 % w/ w for PAMA-C and PAMA-L respectively. The rheological behaviour of these lubricants has been studied in a previous work, in particular the dependence on pressure, temperature and shear rate of their viscosity under ranges representative of the application [1]. Film thickness tests are carried out in pure rolling and isothermal conditions using a ball-on-disk tribometer and differential colorimetric interferometry [2]. Transparent disks has been coated with an additional SiO 2 spacer layer to allow accurate measurements below 100 nm, which represents the limitation of conventional interferometry. Two temperatures (25 and 75 °C), a unique Hertzian contact pressure of 0.5 GPa and entrainment velocities from 0.001 to 5 m/ s are applied, and each result represents the mean value obtained from five distinct measurements. Experimental data are compared with analytical isothermal EHD predictions that have been proven to be accurate [3] and in which the lubricants actual rheological response established in [1] is taken into account. 3 Results and discussion For the base oil, the experimental film thicknesses are in good agreement with the predictions, at 25 and 75 °C, whatever the entrainment speed (U e ). The Newtonian behavior of the base oil is thus confirmed, since it is one of the assumptions to apply EHD prediction. Weak thermal effects occurred at high speed due to the intense shearing at the contact inlet, above 2 and 5.5 m/ s at 25 and 75 °C, respectively. Based on these results, film thickness is measured with polymer solutions up to approx. 200 nm, therefore under isothermal or low-to-medium entrainment speed conditions. The central and minimum film thicknesses at 75 °C are plotted in log-log scale (Figures 1 and 2) for the base oil and the four polymer solutions. Power regressions (not shown here) have been performed on all the experimental curves of central film thicknesses (h c ) against U e , at 25 and 75 °C. It is found that the power exponents (i.e., Relationship between Film Forming Capability and Rheology of Lubricants with VI Improvers Pauline Cusseau, Philippe Vergne, Laetitia Martinie, David Philippon, Nicolas Devaux, Fanny Briand* * Pauline Cusseau Philippe Vergne *) Laetitia Martinie David Philippon Nicolas Devaux Univ Lyon, INSA Lyon, CNRS, LaMCoS - UMR5259, 69621 Villeurbanne, France Fanny Briand TOTAL, Centre de Recherche de Solaize, BP 22, 69360 Solaize, France *Corresponding author: philippe.vergne@insa-lyon.fr Aus Wissenschaft und Forschung / TAE-Plenarvorträge 34 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 the slopes in log-log plots) are very close to the one of the EHD analytical equation as soon as the lubricant film remains equal or thicker than typically 50 nm, this threshold value being highlighted in Figures 1 and 2 by a horizontal dashed line. Beyond this value, the polymer addition does not change the EHD response of the base oil, a shift of the curves to the left may occur at the most. This indicates that everything happens as if the lubricant is more viscous than the base oil, highlighting the thickening function of some polymers. At 75 °C, a weak increase (+5 to +10 %) of hc is found for PAMA-C and PISH-S solutions, whereas larger variations (+20 to +25 %) are obtained with PAMA-L and OCP-LE. Interestingly, the apparent viscosities given by the regressions on h c > 50 nm are consistent [4] with the Carreau- Yasuda predictions obtained based on rheological measurements reported in [1]. Below 50 nm, it is no longer possible to ignore that scale-related features can modify the film thickness capability of the lubricants. Indeed, the dimension and the chemistry of the molecules may become of critical interest. This has been evidenced by Luo et al. [5] who considered the dimension of the lubricant molecules relatively to the film thickness. Here, in the very thin film (VTF) regime, we consider the gyration radius R g to define the geometrical size of a polymer. R g is classically determined from the hydrodynamic radii R h calculated with the Einstein’s law (see [1] for more details). Figure 1: Central film thickness (h c ) versus entrainment speed (U e ) at 75 °C Figure 2: Minimum film thickness (h min ) versus entrainment speed (U e ) at 75 °C Figure 3: Comparison between experimental minimum film thicknesses (dots) and predictions (dotted lines) of PAMA adsorption. h min for the base oil is reported to exemplify a purely Newtonian EHD behaviour. At this stage, PISH-S and OCP-LE solutions are left as they don’t exhibit any specific behaviour apart behaving as non-Newtonian fluids. Third degree polynomial regressions on the PAMA-C and PAMA-L h min results led to the estimation of h’, the thickness of a residual film at U e = 0 m/ s. This is shown in Figure 3 for h min at 75 °C. The orange dotted line suggests a residual-adsorbed lay- Aus Wissenschaft und Forschung / TAE-Plenarvorträge 35 Tribologie + Schmierungstechnik · 67. Jahrgang · 4/ 2020 er of 8.2 nm for the PAMA-C, whereas the red dotted line is plotted for h’ = 17.2 nm for PAMA-L. Those values, compared to R g ones of respectively 8.6 and 9 nm, indicate polymer adsorption with one layer in the first case, and two layers for PAMA-L, the larger adsorption of the latter being very likely due to the presence of a polar dispersant. Similar results are obtained with h c and also at 25 °C. 4 Conclusion From this work on the film forming capability of lubricants with VI improvers, it is shown that: i) in the EHD regime, there is a quantitative agreement between film thickness measurements and predictions, provided that the latter are calculated on the basis of relevant rheological parameters obtained independently of the tribological experiments, ii) in the VTF regime (below 50 nm here), PAMA polymers show a critical thickness from which the film thickness significantly departs from the predictions. Several effects occur: a rather complex non-Newtonian behaviour mixing thickening (by polymer addition) and shear thinning, but also the adsorption of the polymer on the surfaces which can be amplified in the presence of polar compounds. More details can be found in [4]. References [1] Cusseau P., Bouscharain N., Martinie L., Philippon D., Vergne P., Briand F., Rheological considerations on polymer-based engine lubricants: viscosity index improvers versus thickeners - generalized Newtonian models, Trib Trans 2018, 61 (3), p. 437-447 [2] Hartl M., Krupka I., Poliscuk R., Liska M., Molimard J., Querry, M., Vergne P., Thin film colorimetric interferometry, Trib Trans 2001, 44(2), p. 270-276 [3] Wheeler J.D., Vergne P., Fillot N., Philippon D., On the relevance of analytical film thickness EHD equations for isothermal point contacts: qualitative or quantitative predictions? , Friction 2016, 4(4), p. 369-379 [4] Cusseau P., Vergne P., Martinie L., Philippon D., Devaux N., Briand F., Film forming capability of polymerbase oil lubricants in elastohydrodynamic and very thin film regimes, Trib Lett 2019, 67(2), 45 [5] J. Luo, S. Wen, and P. Huang, Thin film lubrication Part I: Study on the transition between EHL and thin film lubrication using a relative optical interference intensity technique, Wear 1996, 194, p. 107