HIGHLIGHTS
• A stable dispersion of molybdenum disulfide in water and other polar solvents was achieved through functionalization with urea. 
• The adduct, known as U-MoS₂, exhibited a lower coefficient of friction in water than molybdenum disulfide.
• An 81% decrease in wear rate was found for U-MoS₂ compared to water while molybdenum disulfide in water reduced wear by 55%.
Molybdenum disulfide is a well-know solid lubricant that has been extensively evaluated in applications such as grease. The reason for this lubricant’s effectiveness is a three-dimensional layered structure, where trilayers consisting of sulfur-molybdenum-sulfur covalent bonds are held together by van der Waals forces. Friction is reduced due to the easy sliding between these layers. 

In a previous TLT article,¹ these properties were utilized by researchers in an attempt to achieve superlubricity when interacting with nanodiamond particles and a diamond-like-carbon surface. Superlubricity was obtained over a long period of time due to the molybdenum disulfide decomposing to its elements (molybdenum and sulfur) under the test conditions leading to the formation of molybdenum carbide with the diamond-like-carbon surface. The sulfur bonded to the carboxyl and oxygenated species present on the nanodiamond surface also provided a lubricity benefit. 

The properties that enable molybdenum disulfide to be so effective in reducing friction in the neat form or in an oil phase also hinder this material’s ability to be effective in an aqueous environment. STLE Past President Dr. Hong Liang, Oscar S. Wyatt Jr. Professor of Mechanical Engineering at Texas A&M University in College Station, Texas, says, “Molybdenum disulfide contains a basal plane that inhibits its ability to form a stable dispersion in a polar solvent such as water. The molecule is not polarizable and does not have a dipole moment due to its planar structure resulting in an inherent instability when placed in polar media. Molybdenum disulfide molecules are more likely to aggregate quickly when in an aqueous dispersion and quickly precipitate out of a suspension due to the van der Waals forces, which are a key to its excellent lubricity characteristics.”

Efforts to functionalize molybdenum disulfide and other hydrophobic molecules to enable them to be more compatible with polar solvents have produced limited success. For example, graphene dispersions functionalized with amino-terminated block copolymers were able to reduce friction by 53% and wear by 91% in water-based systems. A reduction in coefficient of friction was accomplished when molybdenum disulfide, at a 0.1% treat rate, was modified with a thiol functionality.

Liang says, “There are a number of good approaches that have been proposed for functionalizing hydrophobic molecules, but additional research is needed. Several of these approaches show promise but need to be explored further.”

Liang and her colleagues have now developed a unique approach to form stable dispersions of molybdenum disulfide in water and other polar solvents. 

U-MoS₂
The researchers developed a stable dispersion of molybdenum disulfide in water and other polar solvents by functionalization with urea. Liang says, “We originally were trying to figure out a better approach for stabilizing molybdenum disulfide in oil and water. Urea was considered a candidate to meet that objective. But we found out by first oxidizing molybdenum disulfide using hydrogen peroxide, isolating oxidized molybdenum disulfide particles, and then introducing them to urea, an adduct of urea and molybdenum disulfide known as U-MoS₂ can be isolated. The key to the process was first adding oxygen-containing functional groups to the molybdenum disulfide in the oxidation step, which facilitated a successful adsorption reaction with urea.”

Both reactions occurred at ambient temperature, and no byproducts were identified according to Liang, except for unreacted urea. Fourier-transform infrared spectroscopy (FTIR) analysis confirmed the functionalization of molybdenum disulfide with urea.

A 0.1% dispersion of U-MoS₂ in water was evaluated for a period of one month. As shown in Figure 1, the sample on the left, molybdenum disulfide dispersed in water, demonstrates the instability of this material in an aqueous environment. In contrast, the sample on the right, U-MoS₂, exhibits a stable dispersion in water. 


Figure 1. The adduct of urea and molybdenum disulfide forms a stable dispersion in water as shown in the image on the right. In contrast, molybdenum disulfide does not form a stable dispersion in water as shown in the image on the left. Figure courtesy of Texas A&M University.

The tribological properties of U-MoS₂ were evaluated using tribometer with a test mode of linear reciprocating ball-on-plate. The substrate was an AISI 4130 low-carbon steel plate, and the counter material was an AISI E52100 steel ball. Experiments were conducted under the mixed lubrication regime with a sliding speed of 5 centimeters per second and a normal load of 5 newtons.

Coefficient of friction values were measured over 4,000 cycles and showed that U-MoS₂ exhibited a figure of 0.058, which was lower than molybdenum disulfide (0.102) and also more consistent over the course of the test.
 
Under identical test conditions, the wear properties of U-MoS₂ were determined by evaluating wear tracks. Compared to urea, and molybdenum disulfide, the wear track for U-MoS₂ in water was found to be both the shallowest and narrowest of all samples. An 81% decrease in wear rate compared to water was found for U-MoS₂. In comparison, molybdenum disulfide in water reduced wear by 55%. 

Liang believes that the beneficial characteristics of urea and molybdenum disulfide combine to produce the lower coefficient of friction and wear values. She says, “The mechanism for how U-MoS₂ displays such good performance starts with the ability of urea to form hydrogen bonds with water and coordinate with the steel surface. In effect, urea acts as a bridge between the layered structure of molybdenum disulfide and the steel surface improving lubricant film adhesion. The sliding behavior of molybdenum disulfide layers facilitates the formation of a continuous and uniform lubricating film over the steel surface that acts as a protective barrier to further reduce friction and wear.”

Similar results were found by dispersing U-MoS₂ ethylene glycol and glycerol. Liang says, “We evaluated ethylene glycol and glycerol because both solvents potentially can be used as coolants in (electric) vehicles, power generators and various electronic devices.”

Future work will involve gaining a better understanding of the mechanism for how U-MoS₂ displays low coefficient of friction and wear rates. In addition, the researchers will study how other compounds similar in structure to urea interact with molybdenum disulfide.

The ability to utilize molybdenum disulfide in water-based lubricants may now lead researchers to figure out how to exploit this finding in applications where only oil-based fluids have been effective. Additional information can be found in a recent article² or by contacting Liang at hliang@tamu.edu.
REFERENCES
1. Canter, N. (2018), “Generation of long-lasting superlubricity at the macroscale,” TLT, 74 (8), pp. 12-13. Available at www.stle.org/files/TLTArchives/2018/08_August/Tech_Beat_I.aspx.
2. Kabir, M., Dias, D., Marjuban, S., Avais, M., Castaneda, H. and Liang, H., (2025), “Effects of urea-functionalized MoS₂ on hydrophilic lubrication,” Tribology International, 203, 110384.