| Metal-free oil additives containing phosphorus and sulfur are vital constituents of an oil formulation, and have been used for several years to protect engine parts from friction and wear. These chemically active compounds function by adsorbing chemically onto a metallic surface and reacting with it to generate a protective inorganic, sacrificial film. X-ray absorption near-edge structure spectroscopy (XANES) was used to investigate the chemistry of thermal and rubbing films generated on steel from a number of different metal-free oil additives containing phosphorus and sulfur.; In general, phosphate esters reacted with steel to generate thermal and tribochemical films of iron (II) polyphosphate. DPP, a diaryl phosphate ester in mono/diprotic acid formed thicker thermal films at lower temperatures compared with the other neutral phosphate esters used. A correlation was then established between how quickly phosphate esters react with the surface to generate a film and wear protection.; Thermally, organosulfur extreme-pressure (EP) oil additives react with steel to generate a film composed entirely of iron sulfate throughout the film. Rubbing changes the overall chemistry, with several different species of sulfur formed. Under antiwear (AW) conditions, the main species formed was pyrite (FeS2), with the presence of oxidized species including sulfite and sulfate. Topographical properties of the films were monitored using the atomic force microscope. AFM revealed that the AW films were quite heterogeneous with the formation of very small pad-like features. Under EP conditions, removal of the oxide layer present on steel resulted in the formation of only FeS.; The macrochemical properties of ashless thiophosphate derived tribochemical films showed that the phosphorus chemistry is determined within the first few steps of tribofilm formation, while the sulfur chemistry changes over prolonged rubbing times. Photoemission electron microscopy (PEEM) allowed for the acquisition of XANES at the sub-micron level and revealed that for ashless thiophosphates, an ideal AW film is associated with a thicker film that possesses wider pads and regions of longer chain polyphosphates dispersed throughout the tribofilm. Combining ashless thiophosphates with calcium detergents resulted in the formation of short chain calcium phosphate AW films. Surprisingly, the thinner phosphate films that were produced did not result in diminished wear protection. |
