Tribological Properties of Alkylphenyl Diphosphates as High-Performance Antiwear Additive in Lithium Complex Grease and Polyurea Grease for Steel/Steel Contacts at Elevated Temperature

The alkylphenyl diphosphates pentaerythritol tetrakis(diphenyl phosphate) (PDP) and trimethylolpropane tris(diphenyl phosphate) (TDP) were evaluated as the antiwear additives in lithium complex grease and polyurea grease at 200 °C. The results indicated that both additives may effectively reduce the sliding friction and wear as compared to the base greases. The tribological performances were generally better than the normally used molybdenum disulfide (MoS2)-based additive package in lithium complex grease and also in polyurea grease. Boundary lubrication films composed of Fe(OH)O, Fe3O4, FePO4, and compounds containing the P–O bonds were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance.