Theories explaining the way in which liquid-surface interaction affect boundary lubrication are reviewed. In all views, adsorption is a critical step governing the end result—albeit through differing mechanism. This review collects persuasive qualitative experimental support to this effect. A quantitative treatment deriving the friction or wear coefficients as functions of the heats of adsorption of boundary lubricant molecules is discussed in more detail, and recommendations for testing existing models are made. Methods for measuring heats of adsorption are described and the interrelation between the different measurements is explained. Heat of adsorption values predicted from sliding friction and wear measurements are higher than the integral heats of adsorption measured for the lubricant. This discrepancy could mean that a differential heat (i.e., specific for a certain surface coverage) determines the friction and wear characteristics of the lubricant-surface combination or that the surface temperatur...

Liquid Wetting on Metal Surfaces and the Heat of Immersion/Adsorption in Lubrication

We have observed Raman scattering from molecular layers absorbed on the oxide layer in Al-Al2O3-metal tunnel junctions1-6. The observation of these signals was remarkable, since the molecular layers were present only in monolayer coverages, and were separated from the incident laser beam by a typically 20 nm thick metal film. In fact, the Raman scattering could be measured because the observed signal levels were up to ~ 5x104 times more intense per molecule than expected from similar measurements on bulk specimens. Therefore, surface enhanced Raman scattering, analogous to that observed in electrochemical cells and in vacuum (and discussed in other Chapters in this volume), can be present in tunnel junction structures as well. Our experiments conclusively demonstrated the role of electromagnetic resonances in surface enhanced Raman scattering: 1) Junctions with Ag second metal electrodes had the most intense spectra, with Cu and Au ~ 100 times weaker, and no signals were observed for Pb, Sn, In, or Al second metal electrodes. This is consistent with the fact that low imaginary parts of the dielectric response of the metals were required for strong electromagnetic resonances. 2) We found that the observed spectra were most intense for tunnel junctions evaporated on highly roughened substrates. Surface roughness was required to couple the surface electromagnetic resonances to external radiation.

Tunnel Junction Structures

The intensity of Raman scattering from molecules in solution suggests that the observation of Raman scattering from molecular monolayers should be very difficult given the small number of molecules involved. However, recent experimental work on Raman scattering from molecules adsorbed on the surface of metal electrodes in electrochemical cells has shown that strong Raman scattering can be obtained from a monolayer of adsorbed molecules.1,2,3 It has been estimated that the Raman cross section of a pyridine molecule adsorbed on an electrochemically prepared Ag surface in a KC1 solution is six orders of magnitude larger than for the same molecule in solution.2 Although this large enhancement has stirred considerable experimental and theoretical interest, its microscopic origins remain unknown.4,5,6

J. R. Kirtley

Papers

Raman Spectroscopy of Molecular Monolayers in Inelastic Electron Tunneling Spectroscopy Junctions