A study of the tribological behaviour of dialkyldithiophosphate esters as additives in rape seed oil

Zinc dialkyldithiophosphates (ZDDP) cannot be used as additives in biodegradable lubricants because of their zinc content. In investigating substitutes for ZDDP, dialkyldithiophosphate esters have been synthesised, and their tribological behaviour as additives in rape seed oil has been evaluated using a four-ball friction and wear tester and compared with that of ZDDP. The results show that these additives have better antiwear properties and load-carrying capacity than rape seed oil alone. The morphologies and the elemental chemical states on the worn surfaces of the lubricated steel balls of the tester were examined using X-ray photoelectron spectroscopy and scanning electron microscopy. The tribological mechanism is discussed on the basis of the experimental results.