Lanthanum dialkyldithiophosphate (LaDDP) was synthesised. The tribological properties of a combination additive of LaDDP and a n organo-borate (OB) i n oil were evaluated with a fourball test machine. The results show that the combination of LaDDP and OB gives better load-carrying capacity and antiwear and friction-reducing properties than the individual components. Rubbed surfaces after the friction tests were investigated using X-ray photoelectron spectroscopy, Auger electron spectroscopy, and microhardness testing. The results indicate that a boundary film is composed of deposit films (including La,O, and B,O,), chemical reaction films (including FeSO,, FePO,, and FeS), and a tribodiffusion layer (including La and B). Lanthanum can promote the decomposirion of OB and the tribodiffusion of boron. From a mass spectroscopy analysis a tribochemical mechanism is suggested.
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