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 was used to monitor ZDDP antiwear film thickness with rubbing time. Thermal immersion films of varying thickness were generated from the ZDDP and analysed 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 spectra were plotted against PIXE mass thickness values in order to establish calibration curves. Antiwear films were analysed using XANES spectroscopy, and average mass thicknesses were extrapolated from the calibration curves. 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 thickness. A set of antiwear films rubbed in the presence of ZDDP for various lengths of time showed an increase in film thickness, followed by thinning of the film. The decrease in film thickness is believed to be due to wear caused by the ZDDP solution decomposition products acting as an abrasive in the contact region.