Addition of Molybdenum-derived organic compounds (MoOC) to engine oils does not necessarily guarantee the improvement of their fuel economy performance, as their effectiveness is frequently affected by other additives. For these reasons, there is the need to develop more knowledge about the unforeseen effects promoted by other additives on MoOC mechanisms and to develop suitable ways to avoid them.
In the present work, starting with evidence from engine testing, the friction-reducing activity of two different MoOC has been evaluated extensively by tribometry. Triboreactions have also been studied on tribo-layers and on in-situ debris produced in the presence of competing additives, employing the ‘dual-analysis’ technique.
Results demonstrated that various MoOC structures impact differently on boundary friction coefficient, depending on their interactions with other additives. In particular, the investigations on mixed triboreactions have determined that overbased detergents promote antagonistic reactions, with the formation of new MoOC compounds that are different from the effective lamellar MoS2 dispersed in the glassy polyphosphate antiwear layers. With these understanding, it is possible to design more favourable engine oil formulations by reducing the friction coefficient and contributing to improve the final fuel efficiency performance.