Electron spectroscopic observations on extreme pressure lubrication

Abstract

COMPOUNDS of sulphur, phosphorus, zinc and chlorine are added to lubricating oils to control wear under arduous conditions by forming films on rubbing surfaces. The films are extremely thin and frequently amorphous. Only rarely has it been possible to identify compounds present although electron probe microanalysis can show the distribution of elements. We here present new information, obtained from electron spectroscopic measurements, mainly concerning the binding energies of photoelectrons (ejected with AlK_α X rays) the magnitudes of which are characteristic of particular atoms and electrons, and which may therefore be used for analytical purposes. We also made some observations of the kinetic energies of Auger electrons, which are also well defined and of analytical value. These energies depend on the charge on the atom, so that for states of combination of an element where the charges on the atoms are sufficiently different, valuable ‘chemical shifts’ are observed; sulphur is a favourable case with a shift of about 6 eV from sulphide to sulphate. Only electrons that escape without collision are of value, so the analysed depth was limited to a few tens of Å, the region of interest in lubrication.