Effect of Additives on the Replenishment Performances and Vibration Resistance of Polyurea Greases

This study investigates the effects of zinc dialkyldithiophosphate (ZDDP), molybdenum dialkyldithiophosphate (MoDDP), and potassium borate (PB) on the replenishment performance and vibration resistance performance of polyurea grease, using an MTM (Mini Traction Machine) and an optical elastohydrodynamic lubrication (EHL) test rig. Results show that the addition of ZDDP and MoDDP reconstructs the hydrogen bonding network of polyurea thickener and affects the rheological properties of polyurea grease, thereby achieving the molecular mechanism of “promoted diffusion under static conditions and suppressed diffusion under dynamic conditions”. This complementary effect not only rapidly replenishes the grease in the inlet region but also maintains the oil film in the contact region. These additives also influence the grease finger structure, which reduces the backflow resistance of the base oil and strengthens capillary-driven replenishment, thus ensuring a continuous and sufficient supply of lubricant in the contact region. Benefiting from the improved replenishment performance, the thickness of the lubricating film formed under the entrainment effect is maintained at a higher level, which effectively separates the friction pair interface and reduces vibration caused by interface contact. Both additives can also promote the formation of a thickener deposited layer on the raceway surface; this deposited layer helps maintain the integrity of the oil film under starved lubrication conditions, further enhancing the film-forming capability and film thickness retention capability of polyurea grease and ultimately achieving excellent vibration resistance performance.