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

TL;DR

This study investigates how MoDTC and ZDTP additives synergistically influence the nanometer-scale frictional behavior of anti-wear tribofilms, revealing that their combination significantly reduces friction through pressure-induced structural changes.

Contribution

It provides new insights into the combined effects of MoDTC and ZDTP on tribofilm structure and friction, highlighting the pressure-dependent formation of low-friction MoS2 layers.

Findings

MoDTC films exhibit high friction at low pressure.

Combining MoDTC with ZDTP enables high-pressure conditions for low friction.

Low friction domains are linked to MoS2 layer sliding under pressure.

Abstract

The layered structure and the rheological properties of anti-wear films, generated in a rolling/sliding contact from lubricants containing zinc dialkyldithiophosphate (ZDTP) and/or molybdenum dialkyldithiocarbamate (MoDTC) additives, have been studied by dynamic nanoindentation experiments coupled with a simple modelling of the stiffness measurements. Local nano-friction experiments were conducted with the same device in order to determine the evolution of the friction coefficient as a function of the applied pressure for the different lubricant formulations. For the MoDTC film, the applied pressure in the friction test remains low (<0.5 GPa) and the apparent friction coefficient is high ($\mu$ > 0.4). For the tribofilms containing MoDTC together with ZDTP, which permits the applied pressure to increase up to a few GPa through some accommodation process, a very low friction domain appears (0.01 < $\mu$ < 0.05), located a few nanometers below the surface of the tribofilm. This low friction coefficient is attributed to the presence of MoS2 planes sliding over each other in a favourable configuration obtained when the pressure is sufficiently high, which is made possible by the presence of ZDTP.