Inelastic Electron Tunneling Spectra of Lubricant Oil Additives on Native Aluminum Oxide Surfaces

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Abstract
Zinc dithiophosphates (ZnDTPs) have been used extensively as antiwear agents for over 30 years. The mechanism of action of these remarkable materials is not well understood. In particular, the molecular composition of the antiwear film produced by the ZnDTPs remains an area requiring investigation. In this paper, the molecular composition of adsorbed ZnDTPs on aluminum oxide (Al2O3) surfaces has been studied by inelastic electron tunneling spectroscopy (IETS), a highly sensitive molecular spectroscopy specifically applicable to surface species at low coverages. As such, it complements other surface science techniques for determining surface compositions. A vibrational spectroscopy, IETS utilizes the literature on group characteristic frequencies available from the very extensive correlations developed for infrared and Raman spectra. Thus, functional groups may be identified that are formed by the reaction of a lubricating oil additive with a metal surface covered by a native oxide. By IETS, we show that ZnDTP aryl esters are dissociatively chemisorbed, and do not appear to form P = S groups bonded to the surface. By contrast, primary and secondary ZnDTPs retain the phosphate ester groups and bond to the surface by the P = S group to different degrees. These spectral differences are qualitatively correlated with the observed antiwear performance of these materials.