Transfer films and friction under boundary lubrication

The role of transfer phenomena in the mechanisms of friction reduction by organic molybdenum compounds is studied with the aid of ultrahigh vacuum (UHV) analytical tribometry. Additives used are zinc dithiophosphate (Zndtp), molybdenum dithiophosphate (Modtp), molybdenum dithiocarbamate (Modtc) and Modtc/Zndtp combinations. Experiments involve UHV friction tests on tribofilms formed previously and in situ surface analyses by Auger electron spectroscopy (AES) and imaging. In the presence of Modtc, friction reduction was found to be associated with the transfer of highly dispersed MoS2 as isolated sheets, from the tribofilm to the slider. In the presence of Modtp (or the mixture Modtc/Zndtp), a two-step tribochemical reaction is generally observed. First, phosphate from the film is transferred to the oxide on the pin and the friction coefficient is about 0.3, after an induction period of a few cycles, pure MoS2 single sheets are transferred to the pin and the phosphate is eliminated as wear debris from the contact zone. Friction is then at its lowest value (0.02). The chemistry of the transfer phenomena is modeled using the hard and soft acid and base (HSAB) principle as described by Pearson. The overall data suggest that friction behavior under boundary lubrication with additives may be directly related to molecular scale transfer mechanisms in general.