Reactive formation of a few 2D layers of MoS2 using triboactive Mo surfaces and their interaction with anti-wear additives and dispersants

Abstract

In this work, we study the use of triboactive surfaces containing metallic Mo for promoting the in-situ formation of MoS2 in the presence of lubricants containing S. We evidence the in-situ formation of (002)-oriented MoS2 layers that lead to a decrease in friction from 0.3 down to 0.07. High-resolution transmission electron microscopy analyses reveal that the macroscopic reduction in friction is caused by only a few self-assembled 2D-MoS2 monolayers that are formed via a tribochemical reaction between the metallic Mo from the surface and the S from the lubricant. In the absence of S in the lubricant, friction is high and unsteady. We also address the interaction of the in-situ tribochemically formed MoS2 layers with lubricant co-additives, such as anti-wear and dispersants. The reactive in-situ formation of MoS2 using Mo triboactive surfaces can occur in conjunction with the formation of 60 nm thickness Zinc dialkyl dithiophosphate (ZDDP) tribofilms, as revealed by transmission electron microscopy. In contrast to that, the use of dispersants prevents the formation MoS2 and results in a high coefficient of friction. The results obtained contribute to the development of greener lubrication concepts for reducing friction and wear, as they rely on triboactive surfaces for the formation of MoS2 and avoid the use of noxious organo-molybdenum compounds in lubricants.