INELASTIC ELECTRON-TUNNELING SPECTRA OF LUBRICANT OIL ADDITIVES ON NATIVE ALUMINUM-OXIDE SURFACES

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 composi tion of the antiwear film produced by the ZnDTPs remains an area requi ring investigation. In this paper, the molecular composition of adsorb ed ZnDTPs on aluminum oxide (Al2O3) surfaces has been studied by inela stic electron tunneling spectroscopy (IETS), a highly sensitive molecu lar spectroscopy specifically applicable to surface species at low cov erages. As such, it complements other surface science techniques for d etermining surface compositions. A vibrational spectroscopy, IETS util izes the literature on group characteristic frequencies available from the very extensive correlations developed for infrared and Raman spec tra. 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 dissociat ively chemisorbed, and do not appear to form P = S groups bonded to th e surface. By contrast, primary and secondary ZnDTPs retain the phosph ate ester groups and bond to the surface by the P = S group to differe nt degrees. These spectral differences are qualitatively correlated wi th the observed antiwear performance of these materials.