23rd International Colloquium Tribology - January 2022 399 Studying the action of surface active lubricant additives by surface analytical methods T. Rühle BASF SE, Carl-Bosch-Straße 38, 67036, Ludwigshafen, Germany Corresponding author: Thomas.ruehle@basf.com J. Eickworth Fraunhofer IWM MikroTribologie Centrum, Wöhlerstraße 11, 79108 Freiburg, Germany M. Dienwiebel Karlsruher Institut für Technologie KIT, Kaiserstraße 12, 76131 Karlsruhe, Germany 1. Introduction Lubricant formulations typically contain >95 % of a mineral or synthetic base stock and up to 5 % of lubricant additives. Some of these additives are surface active like e.g. corrosion inhibitors or antiwear additives. It is important to understand the action of these additives on the metal surface to be able to develop an optimized formulation for each application. Besides understanding the adsorption behavior of a single additive, it is also important to understand the interaction (synergistic or antagonistic) between two or more additives on a metal surface. The antagonistic interaction between corrosion inhibitors and antiwear additives is a prominent example. Other kinds of additive interactions are described elsewhere [1]. 2. Surface Analytical Methods for Tribology Surface analytical methods are widely used in heterogeneous catalysis to better understand the molecular mechanism of catalytic processes [2]. Since in catalysis as well as in tribology similar systems are considered, a chemical substance adsorbed on a solid, the use of selected analytical tools established in catalysis could also be beneficial to better understand tribological processes. Suitable surface analytical methods are e.g.: • Morphological / Surface Roughness: • White light interferometry (WLI) • Scanning electron microscopy (SEM) • Contact angle / surface energy • Atomic force microscopy (AFM) • Laser fluorescence microscopy (LFM) • Chemical Information: • X-ray photoelectron spectroscopy (XPS) • Diffuse reflectance infrared Fourier transform spectroscopy (DRIFT) • SEM plus energy dispersive x-ray analysis (EDX) • Mass spectrometry (MS) • Secondary ion mass spectroscopy (SIMS) • Thermal stability: • Thermal desorption spectroscopy (TDS) • Calorimetry (DSC) • Gravimetric Methods: • Thermogravimetric analysis (TGA) • Quartz crystal microbalance (QCM) 3. Examples to be addressed by surface analytical methods The surface of machine parts based upon iron alloys typically are terminated by iron oxide and/ or iron hydroxide layers. In this context the question arises how the lubricant additives are coordinated on the surfaces. In principle, there are different modes possible (see Figure 1). Figure 1: Different modes of coordination of additives on iron oxide terminated metals surfaces (Taken from [3]) This kind of chemical information can be derived from XPS spectra. Another question is about the surface coverage of additives on a metal surface. E.g., for friction modifiers it is known, that the coefficient of friction could strongly correlate inversely with the surface coverage (see Figure 2). For this reason, it is of high interest to measure the adsorbed amount of additive and derive the surface coverage from these data. This can be achieved using a highly sensitive gravimetric method like QCM [5]. 400 23rd International Colloquium Tribology - January 2022 Studying the action of surface active lubricant additives by surface analytical methods Another question concerns the question whether friction modifiers are adsorbed as mono-or multilayers on a metal surface. The model of an adsorbed monolayer not always explains the tribological behaviour accurately. Figure 2: Surface coverage (FractCov) of cotton seed oil and the corresponding coefficient of friction (COF) measured in hexane (Taken from [4]) It must be considered that only one or a few layers of friction modifiers on the metal surface could be sheared off over time in the tribosystem. As a consequence, this causes an increase of the coefficient of friction. In a specific example1, the sufficient number of layers to be adsorbed on a surface in order to ensure a stable coefficient of friction over time was determined to be 53 [6]. How the structure of these multilayers might look like is described elsewhere [7] It can be determined by combining the information gained from different methods, like XPS, QCM, MS or SIMS. 4. Ashless dialkyl-dithiophosphate (DTP) + friction modifier In our own study, investigations of 1 % glycerol monooleate (GMO) and 1% of an organic friction modifier (OFM) in combination with an ashless dithiophosphate (DTP) in a mineral oil (MO) and a synthetic base oil (SBO), i.e. poly alpha olefin revealed antagonistic and synergistic effects. Using QCM adsorption measurements, it was possible to determine the amount of adsorbed additive and via the dissipation shift to judge whether the adsorbed layer rather is hard or viscoelastic. By combining with tribometry data, the synergy effect was linked to the adsorption behaviour. In order to get more information, XPS depth profiles for have been measured. Figure 3: Synergistic, intermediate and antagonistic interaction of an ashless DTP and a friction modifier (Taken from [5]) The results are summarized in Figure 3. The combination of dithiophosphate and an organic friction modifier (OFM) revealed a synergistic effect in terms of wear. If the initially formed films are viscoelastic, wear also can be reduced. Also taking data from a XPS depth profile analysis into account, as a mechanism, the adsorption of the OFM on the formed antiwear layer is proposed. 5. Conclusion Surface analytical methods are a powerful tool to better understand tribological processes. The adsorption modes of additives and their action on the surface either as a single additive or in combination/ competition with other additives can be investigated: Lubricant formulators can use these findings in order to further optimize their formulations and therefore save resources. References [1] H.A. Spikes: Additive-Additive and Additive-Surface Interactions in: Lubrication Scienc, Volume 2, Issue 1, pages 3-23, October 1989 [2] J.W. Niemantsverdriet: Spectroscopy in Catalysis - An Introduction; VCH Weinheim (1993). [3] R.M. Cornell, U. Schwertmann: The Iron Oxides - Structure, Properties, Reactions, Occurence and Uses. VCH Verlagsgesellschaft Weinheim (1996) p. 245. [4] G. Biresaw: “Surfactants in Lubrication” in: Lubricant Additives - Chemistry and Applications”, CRC Press, Boca Raton, (2009) 411. [5] L Eickworth, E. Aydin, M. Dienwiebel, T. Rühle, T. Wilke, T.R. Umbach: „Synergistic effects of antiwear and friction modifier additives”, Industrial Lubrication & Tribology, Vol 72, Issue 8, pages 1019-1025 (2020). [6] Gellman, Andrew J. and Spencer, Nicholas D., “Surface Chemistry in Tribology” (2002). Department of Chemical Engineering. Paper 22. http: / / repository.cmu.edu/ cheme/ 22 [7] Crawford, A. Psaila, S.T. Orszulik: “Miscellaneous Additives and Vegetable Oils“ in: R.M. Mortier, M.F. Fox, S.T. Orszulik (Eds.): Chemistry and Technology of Lubricants. Springer Dordecht Heidelberg London New York (2010) S. 189 ff.