Tribology and Rheology of Polypropylene Grease with MoS2 and ZDDP Additives at Low Temperatures

Polypropylene (PP) grease is a type of lubricating grease with excellent low-temperature performance. The wear and friction performance of steel/steel couples lubricated with PP grease containing molybdenum disulfide (MoS2), zinc dialkyldithophosphate (ZDDP) and MoS2/ZDDP as additives at low temperatures was investigated using an Optimol SRV reciprocating tester. Compared with MoS2 or ZDDP as single additives, the combination of MoS2 and ZDDP resulted in outstanding tribological properties, especially for higher-load, longer-duration and low-temperature working conditions. The analysis of the wear surface indicated that MoS2 not only adhered to the steel surfaces to form a solid film, but also combined with ZDDP to form a tribofilm. The active components of the additives reached the metal surfaces effectively, indicating that the polymer system did not interfere with the function of the additives. The rheological experiment results also showed that PP grease with additives can maintain stable viscoelasticity, viscosity recovery rates and ductility at low temperatures.