THE SAM MODEL FOR WEAR INHIBITOR PERFORMANCE OF DITHIOPHOSPHATES ON IRON-OXIDE

Zinc dithiophosphate (DTP) molecules have long been used as wear inhib itor oil additives for automotive engines. In order to obtain an atomi stic understanding of the mechanism by which these molecules inhibit w ear, we examined the geometries, energetics, and vibrations of an oxid ized iron surface [(001) surface of alpha-Fe2O3] using the MSX force f ield (FF) based on ab initio quantum chemistry (QC) calculations, The DTP molecules studied include (RO)(2)PS2 with R = methyl, isobutyl, is opropyl, and phenyl. The alpha-Fe2O3 surface is described using the ge neralized valence bond (GVB) model of bonding. The geometries, binding energies, and vibrational frequencies from ab initio calculations on simple clusters art used with the biased Hessian method to develop the MSX FF suitable for describing the binding of DTP molecules to the su rfaces. We find that the cohesive energies for the self-assembled mono layers (SAM) of the DTP molecules on the Fe2O3 surface correlate with the antiwear performance observed in experimental engine tests. This s uggests that the search for more effective and environmentally benign wear inhibitors can use the cohesive energies for SAM formation as a c riterion in selecting and prioritizing compounds for experimental test ing.