Tribological properties of naphthyl phenyl diphosphates as antiwear additive in polyalkylene glycol and polyurea grease for steel/steel contacts at elevated temperature

Naphthyl phenyl diphosphates: 1-naphthyl diphenyl phosphate (NDP) and 1,5-dihydroxynaphthalene bis(diphenyl phosphate) (DDP) were evaluated as the antiwear additives in polyalkylene glycol and polyurea grease at 200 °C. Results showed that they could effectively reduce the friction and wear of sliding pairs compared with the cases without these additives. Furthermore, the tribological properties of NDP and DDP were generally better than the normally used tricresyl phosphate (TCP) in PAG and molybdenum disulfide (MoS₂) in polyurea grease. Boundary lubrication films composed of Fe(OH)O, Fe ₃O₄, FePO₄ were formed on the worn surface, which resulted in excellent friction reduction and antiwear performance.