The Enhancement of Friction Reduction and Anti-Wear Properties of Polyurea Greases Mediated by a Lithium Salt at Elevated Temperatures

Polyurea grease (PU) is widely used in the lubrication of heavy machinery, but it can still
suffer from structural or performance degradation under extreme conditions such as high
temperatures and heavy loads. This study successfully synthesized a hybrid polyurea
grease (LiTFSI-PU) by incorporating lithium bis(trifluoromethanesulfonyl)imide (LiTFSI)
into polyurea matrix. LiTFSI coordinates with the carbonyl groups (C=O) in the thickener
molecules to form weakly Lewis acidic complex, thereby reinforcing the soap fiber network
structure. As a result, LiTFSI-PU exhibits increased apparent viscosity under shear. The
tribological properties of LiTFSI-PU were evaluated under both ambient and elevated
temperature conditions. At a load of 200 N and 150 ◦C, the average coefficient of friction
for the 3 wt% LiTFSI-PU formulation was 0.094, which is 32.3% lower than that of the
baseline polyurea grease (PU), while the wear volume was reduced by 77.5%. XPS and FIBSTEM/EDS analyses confirmed that LiTFSI-PU forms a multicomponent protective film in
situ during friction, which simultaneously shields the substrate and provides lubrication.
The additive strategy proposed in this work offers novel insights for the development of
high-performance lubricants suitable for extreme thermomechanical conditions.