Hydrophilized MoS₂ as Lubricant Additive

Molybdenum disulfide (MoS2) has been used in a variety of lubrication products due to its highly tunable surface chemistry. However, the performance of MoS2-derived tribofilms falls short when compared to other commercially available antiwear additives. The primary objective of this study is to improve the tribological performance of MoS2 as an additive for lithium-based greases. This was achieved by functionalizing the particle with hydrophilic molecules, such as urea. Experimental results indicate that the urea-functionalized MoS2 (U-MoS2) leads to a notable decrease in the coefficient of friction of 22% and a substantial reduction in the wear rate of 85% compared to its unmodified state. These results are correlated with the density functional theory (DFT) calculation of U-MoS2 to theorize two mechanisms that explain the improved performance. Urea has the capability to reside both on the surface of MoS2 and within its interlayer spacing. Weakened van der Waals forces due to interlayer expansion and the hydrophilicity of the functionalized U-MoS2 surface are catalysts for both friction reduction and the longevity of tribofilms on hydrophilic steel surfaces. These findings offer valuable insights into the development of a novel class of lubricant additives using functionalized hydrophilic molecules.