Resolving the Chemical Variation of Phosphates in Thin ZDDP Tribofilms by X-ray Photoelectron Spectroscopy Using Synchrotron Radiation: Evidence for Ultraphosphates and Organic Phosphates

X-ray photoelectron spectroscopy using a synchrotron source (SR-XPS) with variable photon energy has been used to non-destructively elucidate the variations in surface chemistry from ~5 nm to ~10 nm into the tribofilm derived from zinc dialkyldithiophosphate (ZDDP) in a mineral oil under boundary lubrication conditions. The elemental ratio of P/Zn and “bridging” oxygen (BO)/“non-bridging” oxygen (NBO) decrease as a function of distance from the top surface of the film, suggesting a decrease of the polyphosphate chain-length into the film, as shown in many recent XPS and XANES studies. More importantly, the measured P/Zn ratio of ~3, the BO/NBO ratios of >0.5, the P 2p spectra, and the absence of other balancing cations such as iron, show the first strong evidence for an ultrapolyphosphate (such as ZnP4O11), organophosphates along with other Zn polyphosphates. The existence of ultraphosphates and/or organophosphates in this film appears to be the long-awaited answer to the apparent deficiency of cations in these antiwear films.