23rd International Colloquium Tribology - January 2022 137 Low friction with polymer friction modifier in steel/ steel contact: a combined tribology and physico-chemical approach Nasrya F. Kossoko Université de Lyon, Ecole Centrale de Lyon, Laboratoire deTribologie et Dynamique des Systèmes LTDS, CNRS-UMR 5513, F-69134, Ecully, France Clotilde Minfray Université de Lyon, Ecole Centrale de Lyon, Laboratoire deTribologie et Dynamique des Systèmes LTDS, CNRS-UMR 5513, F-69134, Ecully, France Michel Belin Université de Lyon, Ecole Centrale de Lyon, Laboratoire deTribologie et Dynamique des Systèmes LTDS, CNRS-UMR 5513, F-69134, Ecully, France Benoît Thiébaut Total Marketing & Services, Centre de Recherche de Solaize, Chemin du Canal, BP 22, 69360, Solaize, France Frederic Dubreuil Université de Lyon, Ecole Centrale de Lyon, Laboratoire deTribologie et Dynamique des Systèmes LTDS, CNRS-UMR 5513, F-69134, Ecully, France Corresponding author: frederic.dubreuil@ec-lyon.fr 1. Introduction Polymer Friction Modifier (PFM) are interesting alternatives to classical lubricant friction modifiers but their tribological performance remains to be clearly demonstrated and understood (1). Up to now the development of PFM in automotive lubricant was mainly supported by chemical modification performed on viscosity index modifier molecules like polyacrylate derivatives (2). A breakthrough has been recently achieved by the synthesis of new diblock polymeric systems with a major chemistry change (3). This additive is a diblock polymer with PIB and PEG moieties patented by Croda (4). This work aims to study the friction behavior of this diblock PIB-PEG PFM blended to PAO 4 base oil in steel/ steel contact under severe lubrication conditions and to clarify the action mechanism of the PFM in the contact. 2. Low friction coefficient with a new PFM under boundary lubrication In a previous work (5) we have investigated the action mechanism of this PFM via the combination of multiple tribological tests (steel/ steel contact pin on disc reciprocating and MTM) and various surface characterisation techniques (ToF-SIMS and SFM). Under severe lubrication conditions (classically Pmax = 1 GPa, v = 3mm/ s) and various temperatures we have established that for both MTM and reciprocating tests, solutions containing 1% wt PFM (in PAO 4 base oil) systematically reduce the friction compared to base oil. High temperature measurements (100°C) leads to much lower friction coefficient (0.03) than room temperature ones (cf Figure 1). The performance of this PFM is even better than MoDTC a classical friction modifier. Compared to reciprocating conditions, measurements performed on a MTM at various sliding to roll ratio give the same trends with no impact of the SRR, nor the rubbing speed on the results obtained for complete experiments (including Stribeck curves as preand post-rubbing steps) Fig. 1: Friction coefficient under Pmax = 1GPa, v = 3mm/ s at 25 and 100°C. 138 23rd International Colloquium Tribology - January 2022 Low friction with polymer friction modifier in steel/ steel contact: a combined tribology and physico-chemical approach Optical analysis performed on disc trace after MTM experiment evidenced a film of additive at the steel surface. To understand the way the PFM is trapped ToF-SIMS depth profiles have been performed inside and outside friction scars. Negative ions observed at the surface indicates that the fragment coming from linker between PIB and PEG is the closest part of the molecule to the iron surface (red peak on Figure 2). Fig. 2: Depth profile of the following species C 2 H 3 O- (black) , C 3 H 3 O 2 - (yellow) , CHO 2 - (red) ,C 8 H- (green) and Fe- (blue) , Fe 2 O 3 - (purple) 3. Influence of rubbing conditions Although there is no doubt on the efficiency of this PFM to reduce friction compared to pure base oil, the behaviour of the additive is dependent on the tribological test and conditions. MTM and Reciprocating test are not always similar. Indeed reciprocating experiments show a large variability on the final friction coefficient that could be analysed in terms of two states with “low” or “high” friction coefficient (4). Furthermore, a minimum value of the travel length is mandatory to access stable low friction values. We can conclude that the adsorbed layer of PFM in the contact is fragile and difficult to build under reciprocating tests. The alternate move of the bead on the plane might be at the origin of this behaviour. On the other hand we have a high reproducibility on the MTM apparatus. Classical MTM experiments are divided into 3 parts first a Stribeck curve, then a rubbing phase and finally another Stribeck curve. During this experiment the movement of the bead in contact differs a lot from reciprocating test in terms of rolling, sliding distance and speed. 4. Step by step MTM Test analysis The influence of each steps as been studied to emphasize the difference between MTM and reciprocating test. Various experiments have been conducted to isolate and combine the steps and their contribution to the final frictions results. Interestingly it appears that the first Stribeck curve have a big influence on the tribological behaviour of the PFM: during the rubbing phase in the boundary regime (1 GPa, 3mm/ s) a transient period where the friction coefficient is gently reduced can be evidenced. This induction time is absent when a first Stribeck curve has been performed. Furthermore, without a first Stribeck curve, one can notice an impact of the SRR during the rubbing step and the final friction coefficient. For a better understanding of Stribeck curve’s impact on the surface changes, post processing imaging have been done on friction scars by in situ scanning force microscopy (cf Figure 3). Fig. 3: Topographic image (10x10 mm) of a MTM trace realized in PAO4 in tapping mode after a Stribeck curve. On figure 3, one can notice some big patches of additive in the sliding trace after a first Stribeck curve at 150% SRR, v= 3mm/ s, Pmax = 1 GPa. Those patches of PFM aggregates are not always present after rubbing test and quite mobile at the surface. They are never present with reciprocating experiments. Those aggregates shows the feeding impact of the contact during the Stribeck curve were the rubbing distance is quite long and initial condition not to severe. Preliminary results on some sub-conditions during the Stribeck curve (speed range, rubbing distance, SRR value) to get stable low friction coefficient will be presented. 5. Conclusion In this presentation we have deeply studied the action mechanism of a new polymer friction modifier. Under boundary conditions this additive is quite efficient at high temperatures. However the film adsorbed at the interface is fragile, a good supply of the contact interface in PFM is necessary to achieve low friction coefficients. 23rd International Colloquium Tribology - January 2022 139 Low friction with polymer friction modifier in steel/ steel contact: a combined tribology and physico-chemical approach References [1] Guegan J, Southby M, Spikes H. Friction Modifier Additives, Synergies and Antagonisms. Tribo Lett 2019; 67: 83 [2] Fan J, Müller M, Stöhr T, Spikes HA. Reduction of Friction by Functionalised Viscosity Index Improvers. Tribo Lett 2007; 28: 287-98 [3] Hobday I, Eastwood J. Friction Modifiers for Next Generation Engine Oils. Lube Mag 2014: 27-34 [4] Chen et al (Croda) - Patent WO2015065801A1 [5] Kossoko N.F, Dubreuil F, Thiébaut B, Belin M and Minfray C. Diblock polymeric friction modifier (PFM) in the boundary regime: Tribological conditions leading to low friction Tribo. Int. 2021; 163: 107186.