Application of soft X-ray absorption spectroscopy in chemical characterization of antiwear films generated by ZDDP Part I: the effects of physical parameters

Abstract

X-ray absorption near edge structure (XANES) spectroscopy has been used to study the chemical nature of the antiwear films generated on steel surfaces using zinc dialkyldithiophosphates (ZDDPs). The spectra were recorded using total electron yield (TEY) and fluorescence yield (FY) to investigate the chemical nature of P, S, Ca, O and Fe on the surface and in the bulk, respectively. In the first part of this study, the effects of physical parameters on the composition and mechanism of antiwear film formation is discussed. It has been found that lower concentration of ZDDP, higher temperature and higher load all increase the rate of ZDDP decomposition; and longer rubbing time, higher concentration of ZDDP, moderate temperature, higher load and smooth surfaces help to form long chain polyphosphates. The sulphur in the film in most of the cases is in the reduced form. The presence of sulphate in very short rubbing times or high temperatures has also been detected. When the spectra from the TEY mode and FY mode were compared, a layered structure was found in most of the films. In these films, there is a longer chain polyphosphate on the topmost surface and a shorter chain polyphosphate in the bulk. At short rubbing times and low temperature, unchanged ZDDP is also present in the film. Depth profiling using X-ray photoelectron spectroscopy showed that the antiwear film formed in 30 min is thinner compared with a 12 hour film. Based on the above information, a new mechanism for antiwear film formation is proposed.