Friction‐induced two‐dimensional solid films from lubricant additives

Abstract

Tribochemical interactions between some lubricant additives, namely zinc dithiophosphate (ZnDTP), an antiwear additive, molybdenum dithiocarbamate (MoDTC), a friction modifier, and overbased calcium borate (OCaB) detergent, have been investigated. The nature of the tribofilms formed was studied by combining high‐resolution transmission electron microscopy of wear fragments and inside wear scars with micro‐spot X‐ray photoelectron spectroscopy at the same location of the wear track (‘dual’ analysis). The OCaB/ZnDTP/MoDTC ternary system provides both a low wear rate and ultralow friction, while adding detergent and anti‐corrosion properties to the formulation. The analytical data indicate that such a synergistic effect can be attributed to an outstanding feature of the tribofilm nanostructure: it is composed of perfectly oriented MoS2 single sheets embedded in a single‐phase calcium and zinc borophosphate glass. Compared to phosphate alone, MoS2 sheets are oriented by the borophosphate phase. This could be related to a template effect of friction‐aligned planar molecules of the glassy borate polymer.

Synergistic Effects of MoDTC and ZDTP on Frictional Behaviour of Tribofilms at the Nanometer Scale

“…Thus, the easy sliding of the MoS 2 sheets could result from a favourable orientation induced by sufficiently high contact pressure values. The ability of MoS 2 sheets to orient in a favourable direction was reported by Grossiord et al [31] and Martin et al [32], who recently investigated tribochemical interactions between ZDTP, MoDTC and OCB (overbased detergent calcium borate) additives. Using high resolution TEM observations of wear debris, coupled with wear scar micro-spot XPS analysis, they observed perfectly oriented MoS 2 sheets, with their basal plane parallel to the flaky wear fragments.…”

Mechanical Properties of Molybdenum Disulfide and the Effect of Doping: An in Situ TEM Study

“…These two studies probe the material at the atomic level providing useful information for the study of exfoliation of graphene like materials. Ex situ studies on MoS 2 tend to suggest the validity of these studies, as crystallites have been observed to become oriented by the shear forces experienced in tribological wear. , Doping of MoS 2 with other transition metals is also of interest and is known to increase the wear resistance of the material without dramatic loss of superlubricity . The effect this transition metal inclusion has on the mechanical failure mode of MoS 2 is unknown.…”

Influence of temperature and ZDDP concentration on tribochemistry of surface-capped molybdenum sulfide nanoparticles studied by XANES spectroscopy

“…It is assumed that the MoDTC additive reduces friction by forming a tribofilm containing MoS 2 , which lowers the shear strength of the tribofilm, while the ZDDP additive plays a role in enhancing wear resistance and promoting the formation of MoS 2 [12,[18][19][20]. In the case of molybdenum dithiocarbamate, the high-resolution electron microscopy (HRTEM) study performed on wear fragments have shown that MoS 2 is formed in the contact by decomposition of the additive molecule [21,22].…”