Chemical characterization of antiwear films generated on steel by zinc dialkyl dithiophosphate using X-ray absorption spectroscopy

Abstract The addition of antiwear and extreme-pressure agents to lubricants produces protective films on the sliding surfaces. Two of the most common elements in antiwear/extreme-pressure additives are sulphur and phosphorus. These are commonly added in the form of Zinc dialkyl dithiophosphate (ZDDP). P and S L-edge X-ray absorption near-edge (XANES) spectra of the films and several model compounds containing both S and P have been recorded at high resolution, utilizing synchrotron radiation. Using the spectra of the model compounds as fingerprints, it has been possible to speciate the nature of the antiwear/extreme-pressure films. It has been found that the chemical state of phosphorus in the films corresponds closely to that of polyphosphates and depends on the alkyl group used in ZDDP. Sulphur is present mostly in the form of sulphide.

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