The use of X-ray absorption spectroscopy for monitoring the thickness of antiwear films from ZDDP

X-ray absorption near edge structure (XANES) spectroscopy at the P K-edge w as used to monitor ZDDP antiwear film thickness with rubbing time. Thermal immersion films of varying thickness were generated from the ZDDP and analy sed using XANES spectroscopy and the particle induced X-ray emission (PIXE) technique. P K-edge XANES edge jumps and (1s --> np) peak heights of the s pectra were plotted against PIXE mass thickness values in order to establis h calibration curves. Antiwear films were analysed using XANES spectroscopy , and average mass thicknesses were extrapolated from the calibration curve s. A set of antiwear films formed in the presence of ZDDP and then further rubbed in base oil (no ZDDP) showed no significant decrease in film thickne ss. A set of antiwear films rubbed in the presence of ZDDP for various leng ths of time showed an increase in film thickness, followed by thinning of t he film. The decrease in film thickness is believed to be due to wear cause d by the ZDDP solution decomposition products acting as an abrasive in the contact region.