Tribochemical interactions between Zndtp, Modtc and calcium borate

Tribochemical interactions between antiwear zinc dithiophosphate (Zndtp), f riction 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 f ormed was studied by coupling high-resolution TEM on wear fragments and ins ide-wear-scar, micro-spot XPS in the same location of the wear track (so-ca lled dual analysis). The performance of the Modtc/Zndtp mixture is mainly d ue to the generation of MoS2 single sheets and the digestion of MoO3 in the zinc polyphosphate glass formed. The final result of the tribochemical rea ction is a two-phase tribofilm composed of (i) non-oriented MoS2 sheets (fr iction modifier) embedded in a carbon-rich phase and (ii) a mixed Zn/Mo pol yphosphate glass (antiwear). The Modtc/OCB mixture has a similar antiwear m echanism except that the oxide is not completely eliminated, due to the sof ter action of borate anion compared with phosphate one. Compared to the dat a 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 w ear rate and an ultralow friction value, while adding detergent and anti-co rrosive properties to the formulation. Our analytical data indicate that th e synergistic effect can be attributed to an outstanding nanostructure of t he tribofilm formed. It is composed of a single-phase material containing p erfectly oriented MoS2 single sheets embedded in a calcium and zinc boropho sphate 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 o riented in the borophosphate could be related to aligned molecules of the g lassy polymer in the direction of sliding.