Eco-friendly lubricant additives are of primary environmental and economical importance. Their design rely on our understanding of anti-wear additives such as the zinc dialkyldithiophosphate (ZDDP). ZDDP has the remarkable property of forming a thin protective coating against wear, called ZDDP-tribofilm, at the interface of a boundary lubricated contact. However, detailed reactional pathways leading to the ZDDP-tribofilm formation and mechanisms responsible for wear inhibition are still unsolved. High local pressures in the range of several giga pascals in a boundary lubricated contact are likely to modify the atomic structure and macroscopic properties of materials at this interface. Our work focuses on an in situ analysis of structural modifications of ZDDP-like model compounds when exposed to high hydrostatic pressures, up to 20 GPa. We chose to model the ZDDP additive by zinc phosphate compounds (ZP) after performing tribological tests under boundary lubrication conditions using colloidal Zn3(PO4)2, Zn2P2O7 and Zn(PO3)2 compounds mixed to a poly-alpha-olefin oil, at ambient temperature. Formation of a tribofilm is observed at the interface for Zn3(PO4)2 while it is negligible for Zn2P2O7 and Zn(PO3)2 materials. Mechanical and topographical properties of the amorphous film formed from Zn3(PO4)2 at the interface, show some similarities with ZDDP tribofilm. In situ micro-Raman spectroscopy was used to probe PO4 tetrahedra structural modifications in ZP compounds of different phosphate chain lengths, up to 20 GPa. Pressure had no effect on PO4 tetrahedra polymerization in tested materials and cannot account for the formation of long-chain phosphates found in the ZDDP-tribofilm. Between 0.1 MPa and 4 GPa, α-Zn3(PO4)2 Raman spectra undergo important modifications which were attributed to a phase transition of the initial crystalline structure. Above 4 GPa, Zn3(PO4)2 Raman spectra show a progressive loss of the long-range order in the sample. The structure recovered after de- compression is similar to an amorphous Zn3(PO4)2 compound. Local environment of Zn atoms in α-Zn3(PO4)2 was investigated under high static pressure by in situ Zn-K edge EXAFS and XANES spectroscopies. Crystalline order around Zn atoms was found to vanish down to the second sphere of coordination and the mean Zn-O bond length is increased by 0.046 ± 0.005 A when pressure increases from 0.1 MPa to 3.5 GPa. The coordination number around Zn atoms does not change significantly at a maximal pressure of 7 GPa. The disordered local structure of zinc atoms observed at high pressure is conserved at decompression. Finally, a reciprocating tribometer combined with in situ micro-Raman spectroscopy was used to probe α-Zn3(PO4)2 structural transformations occurring under combined shear and pressure constrains at ambient temperature. Tribologically constrained Zn3(PO4)2 structures were found to be identical to high pressure phases formed in static high pressure experiment and led to the formation of an amorphous zinc orthophosphate tribofilm
In situ analysis of anti-wear mechanism of zinc phosphates