Tribochemical interactions between Zndtp, Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), friction modifier molybdenum dithiocarbamate (Modtc) and overbased detergent calcium borate (OCB) lubricant additives have been investigated. Friction tests were performed in mild wear conditions under boundary lubrication, in order to enhance tribochemical surface effects. The nature of tribofilms formed was studied by coupling high-resolution TEM on wear fragments and inside-wear-scar, micro-spot XPS in the same location of the wear track (so-called dual analysis). The performance of the Modtc/Zndtp mixture is mainly due to the generation of MoS2 single sheets and the digestion of MoO3 in the zinc polyphosphate glass formed. The final result of the tribochemical reaction is a two-phase tribofilm composed of (i) non-oriented MoS2 sheets (friction modifier) embedded in a carbon-rich phase and (ii) a mixed Zn/Mo polyphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear mechanism except that the oxide is not completely eliminated, due to the softer action of borate anion compared with phosphate one. Compared to the data obtained with binary combinations (Modtc/Zndtp, Modtc/OCB and Zndtp/OCB), we show here that the ternary system Modtc/Zndtp/OCB provides both a low wear rate and an ultralow friction value, while adding detergent and anti-corrosive properties to the formulation. Our analytical data indicate that the synergistic effect can be attributed to an outstanding nanostructure of the tribofilm formed. It is composed of a single-phase material containing perfectly oriented MoS2 single sheets embedded in a calcium and zinc borophosphate glass. The ternary system produces a smart material in the interface, because both functions (antiwear and friction reduction) are correlated. Compared to phosphate alone, the mechanism by which MoS2 sheets have been oriented in the borophosphate could be related to aligned molecules of the glassy polymer in the direction of sliding.