The tribological behaviors of some rare earth complexes as lubricating additives Part 2—The antiwear and extreme pressure properties in lithium grease

Abstract

The antiwear and extreme pressure (EP) properties of rare earth 8-hydroxyquinolinates and of rate earth di- n-hexadecyldithiophosphates as additives in lithium grease were evaluated on an Optimol SRV friction and wear tester. The morphologies of the worn surfaces and the elemental distributions in the worn surfaces were observed and determined with an electron probe microanalyzer. Then the action mechanisms of these compounds as additives in lithium grease were discussed. It was found that rare earth di- n-hexadecyldithiophosphates in lithium grease exhibited better antiwear and extreme pressure properties than zinc di- n-butyldithiophosphate (ZDDP), while rare earth 8-hydroxyquinolinates registered antiwear and extreme pressure properties comparable to ZDDP. Moreover, the different tribological behaviors of the compounds as additives in lithium grease were closely related to different mechanisms. That is, rare earth 8-hydroxyquinolinates as additives in lithium grease exhibited physisorption action only, while copper 8-hydroxyquinolinate in lithium grease functioned by both physisorption and coordination ligand exchange reaction. The tribological behaviors of ReDDP was closely related to tribochemical reaction.