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DFT – Modeling of the Reaction of a Polysulfur Extreme-PressureLubricant Additive on Iron SurfaceB. Monasse, P. MontmitonnetEcole des Mines de Paris PARISTECH, CEMEF, UMR 7635, BP 207, 06904 Sophia-Antipolis Cedex, FranceURL: www-cemef.cma.fr e-mail: bernard.monasse@ensmp.fr e-mail: pierre.montmitonnet@ensmp.frABSTRACT: sulfur-containing molecules are used as extreme pressure additive for low-alloy steel surfaces.These molecules react with ferrous surfaces and form FeS x compounds which strength the surface and reducescuffing. The stable conformation and the chemical stability and reactivity of an alkyl sulfur molecule(DTDP) with oxygen and an iron surface are here predicted by DFT model. The effect of the conformation ofthe molecule respective to the surface acts on the surface reactivity.Key words: polysulfur, iron, reactivity, semi-empirical functional,1 INTRODUCTIONSulfur-containing organic molecules have been usedfor a long time as extreme pressure additives inlubricating oils. Several papers report oncomparisons of the tribological efficiency of organosulfuradditives as a function of number of sulfuratoms, alkyl- or aryl-chains [1]. In tribotests,involving ferrous materials and S-containingadditives, the formation of the FeS x compound hasbeen detected in superficial layers by differentsurface analysis techniques [2, 3]. FeS, also knownas troilite, has a layered hexagonal structure, lowshear strength and high melting point (1100 °C), soit is an effective solid lubricant like graphite andMoS 2 [4]. HSAB theory may also shed light on thosespecies which react to form these layers [5]. Themolecule may be thermally decomposed or oxidizedin the lubricant during rolling process or reacts onthe metallic surface. The details of the reaction pathare nevertheless poorly known, and so is the effectof the environment (dissolved oxygen, temperature,…) on preferred reaction paths and reactivity andwill be analyzed in the present work.Quantum models have been applied to study thereactivity of some sulfur molecules on variousmetallic surfaces [6]. Most of the papers are focusedon copper, palladium and nickel for catalyticpurpose with simple sulfur molecules. Reactions acton the first layers of metal mainly for gas phasereactions [6,7]. The sulfur may be randomlydistributed on copper surface or organized asmobile sulfur metal clusters Cu 3 S 3 [8]. Theorganisation of metal sulfur molecules may dependon the reaction path of the sulfur during its reactionwith metal surface. We consider here the reaction ofa di-tertio-dodecyl-pentasulfur molecule (C 12 H 25 S 5C 12 H 25 , DTDP) with a bcc iron [001] surface.2 SIMULATIONThe simulations have been done with a semiempiricalmodel MOPAC using the functional PM5.The results were confirmed by simulations withother semi-empirical functionals AM1 and PM3 anda DFT functional B88-LYP. The MOPAC softwareis coupled to the graphical user interface CAChe.Full geometry optimization of a standalone moleculeis searched first to define the equilibriumconformation of the molecule. This conformation issearched from various initial non-equilibrated state.A molecular dynamics simulation allows us tofollow the conformational change around theequilibrium conformation. The stable conformation