Effects of urea-functionalized MoS2 on hydrophilic lubrication

Abstract

The challenge of achieving effective lubrication in polar lubricants like Water, Ethylene Glycol (EG), and Glycerol has driven the search for advanced lubricant additives. Traditional MoS₂, while widely used in nonpolar lubricants, suffers from poor dispersion stability and inconsistent performance in polar solvents due to its hydrophobic nature. This study addresses those issues by functionalizing MoS₂ with urea (U-MoS₂) to improve its compatibility with polar lubricants. U-MoS₂ was synthesized through a two-step process involving oxidation followed by urea functionalization. The results show that U-MoS₂ significantly enhances dispersion stability, maintaining a uniform suspension for over 30 days. Rheological analysis revealed that U-MoS₂ reduced the viscosity of Water, while in EG and Glycerol, it caused a slight increase in viscosity across the temperature range of 25 to 80 °C. Tribological experiments revealed a substantial reduction in the coefficient of friction (COF) for U-MoS₂-enhanced lubricants. In Water-based systems, the COF was reduced by 84 %, dropping to 0.057 with U-MoS₂. In EG and Glycerol-based systems, the COF reductions were 41 % and 38 %, respectively. Additionally, wear rate analysis demonstrated an 81 % reduction in wear for Water after introducing U-MoS₂, a 51 % reduction for EG, and a 69 % reduction for Glycerol. Wear surface analysis first revealed the presence of a consistent MoS₂ film on the contact surfaces, contributing to the observed reduction in wear and friction. These outcomes suggest that the urea molecule plays a key role in enhancing the adhesion of MoS₂ to the substrate and facilitates the formation of a stable lubricating layer. By addressing the limitations of traditional MoS₂ in polar solvents, this research aims to understand how polar functionalization can improve the performance of MoS₂ in hydrophilic lubrication, potentially expanding its applications across various industrial sectors.