Investigation of the friction and wear behaviors of Cu(I) and Cu(II) dioctyldithiophosphates as additives in liquid paraffin

Organic copper salts have been found to be effective additives for lubricating oils owing to their multifunctional performance. However, little attention has been paid to the comparative investigation of the behaviors of Cu(I) and Cu(II) organic salts as lubricating oil additives. In this paper, both Cu(I) and Cu(II) dialkyldithiophosphates were synthesized and their friction-reducing and antiwear behaviors in liquid paraffin were investigated using a four-ball machine. As a comparison, the friction and wear behaviors of zinc dioctyldithiophosphate (ZDDP) as additive in the same base stock were also investigated under the same conditions. The results showed that the synthesized Cu(I) and Cu(II) dioctyldithiophosphates as additives in liquid paraffin were effective in reducing friction and wear, and the latter is superior to the former in increasing the antiwear ability. The chemical states of some typical elements in the worn surfaces were investigated using X-ray photoelectron spectroscopy (XPS). The morphologies of the worn surfaces and the distributions of some typical elements in the worn surfaces were analyzed with an electron probe micro-analyzer (EPMA). The action mechanism of the synthesized compounds as additives in liquid paraffin were then proposed based on the XPS and EPMA analytical results.