Transfer films and friction under boundary lubrication

The role of transfer phenomena in the mechanisms of friction reduction by o rganic molybdenum compounds is studied with the aid of ultrahigh vacuum (UH V) analytical tribometry. Additives used are zinc dithiophosphate (Zndtp), molybdenum dithiophosphate (Modtp), molybdenum dithiocarbamate (Modtc) and Modtc/Zndtp combinations. Experiments involve UHV friction tests on tribofi lms formed previously and in situ surface analyses by Auger electron spectr oscopy (AES) and imaging. In the presence of Modtc, friction reduction was found to be associated with the transfer of highly dispersed MoS2 as isolat ed sheets, from the tribofilm to the slider. In the presence of Modtp (or t he mixture Modtc/Zndtp), a two-step tribochemical reaction is generally obs erved. First, phosphate from the film is transferred to the oxide on the pi n 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 pho sphate 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 mod eled using the hard and soft acid and base (HSAB) principle as described by Pearson. The overall data suggest that friction behavior under boundary lu brication with additives may be directly related to molecular scale transfe r mechanisms in general. (C) 2000 Published by Elsevier Science S.A.