Phosphate Esters, Thiophosphate Esters and Metal Thiophosphates as Lubricant Additives

Purpose：This review summarizes more than 50 years of phosphate ester, thiophosphate ester and metal thiophosphate use as lubricant additives, with emphasis on antiwear and extreme-pressure mechanisms.

System and methods：It covers alkyl phosphates, triaryl phosphates, thiophosphates and zinc dialkyldithiophosphates, including tribofilm formation on iron and steel and the coupling of oxidation and boundary lubrication.

Key findings and mechanism：The review covers alkyl phosphates, triaryl phosphates, thiophosphates and ZDDP, materials used in lubrication for more than 50 years. It focuses on tribofilm formation on iron and steel and on how temperature, pressure and shear change antiwear and extreme-pressure performance. Phosphate groups adsorb through polar interactions and react under friction-generated heat and shear to form phosphorus-containing boundary films. Thiophosphates and metal salts participate further in tribochemistry, reducing direct metal contact, wear and seizure under extreme pressure.

Application value and limits：The paper provides background on phosphate adsorption and tribochemistry rather than a direct reactive-monomer formulation, making it useful for understanding phosphate-metal interfacial interactions. This is a lubricant-additive review, not a study of polymerizable phosphate monomers. It is useful for phosphate-metal adsorption and tribochemistry, but lubricant dosage and mechanisms cannot be transferred directly to crosslinked coatings or adhesives. Environmental limits on phosphorus, sulfur and heavy metals also require separate evaluation.