Chemistry of surface films from ZDDP by x-ray absorption spectroscopy

Zinc dialkyldithiophosphates (ZDDPs) are an important class of engine oil additives and have been used in engine oil formulations for over 50 years. ZDDPs are added to engine oils for their antioxidant and antiwear properties. They function as antiwear additives by forming a protective (antiwear or tribochemical) film on surfaces in sliding contact. X-ray absorption spectroscopy, in particular, X-ray Absorption Near Edge.Structure (XANES) spectroscopy, was mainly used to characterize the films because of its high resolution, low detection limit and non-destructive characteristics. XANES spectroscopy was shown to be much more chemically sensitive than X-ray Photoelectron Spectroscopy (XPS).; Antiwear film formation on aluminum was investigated. Polyphosphate films were found to form on aluminum alloys, but at lower loads and temperatures than on steel. Polyphosphate thermal films also formed on the aluminum alloys.; The stability of tribochemical and thermal films was investigated. Tribochemical films on steel are very stable in that a film remains on the surface even after rubbing in base oil for 24 hours. Thermal films are less stable. Best wear results were obtained when the coupon and pin were coated with a thermal film and then rubbed in base oil containing ZDDP.; Aryl ZDDP derived tribochemical films contained long chain polyphosphate throughout the film, whereas alkyl derived films exhibited a layered structure with long chain on the surface and short chain in the bulk. Polyphosphate chain length distributions varied between films derived from neutral and basic ZDDPs. XANES analysis of thermal films generated on steel substrates in oil solution showed that ZDDPs decompose at much lower temperatures than in the solid form, in air. An increase in temperature changed the proportion of short chain to long chain polyphosphate. This observation lead to a time dependent study of the thermal decomposition of ZDDP in solution which showed that solution chemistry has an effect on antiwear and thermal film chemistry and thickness. A rearranged ZDDP, or linkage isomer was proposed as an intermediate in film formation. As a result, an earlier film formation mechanism was modified to take the linkage isomer into account. Short chain polyphosphate films formed on steel surfaces from completely decomposed ZDDP oil solutions. A mechanism involving the deposition of short chain colloidal polyphosphate material was proposed.