Properties of tribofilms formed with ashless dithiophosphate and zinc dialkyl dithiophosphate under extreme pressure conditions

Tribological and nanoscale properties of tribofilms from ashless dialkyl dithiophosphates (DDPs) were compared with zinc dialkyl dithiophosphates (ZDDPs) in this study. The tribosurfaces generated from these chemistries were examined using scanning electron microscopy and energy dispersive spectroscopy. The wear debris harvested from the wear tests was examined in a transmission electron microscope in bright field mode with selected area diffraction to examine the crystallinity and nature of the debris particles. Thickness of the tribofilms was measured using the focused ion beam cross-section microscopy indicating that the tribofilms formed by ashless dithiophosphates are significantly thicker than those formed by ZDDP. Nano-mechanical properties of the tribofilms including hardness and reduced modulus, scratch and nano-wear performance of the ashless dithiophosphate were compared to the secondary ZDDP. The overall results indicate that the wear performance of the ashless DDP are comparable or better than when ZDDP is used. The analytical techniques indicate that the tribofilms formed with ashless DDPs are thicker than those formed from ZDDP; however, the hardness and modulus of the tribofilm are lower in the case of ashless DDPs. XANES analysis indicate that in both tribofilms, phosphates are present as short chain phosphates. Ashless DDPs yield iron phosphates while ZDDP yielded a mix of iron and zinc phosphates. Both tribofilms have greater proportion of phosphates compared to sulfur species. The sulfur near the surface is mostly in the form of sulfates while the material in the interior is a mixture of sulfates and sulfides.