A study of the tribological behaviour and action mechanism of thiophosphates in rape seed oil

Tri-n-butyl phosphate and two thiophosphates, tri-n-butyl thiophosphate and tri-n-octyl thiophosphate, were synthesised. The tribological properties of the compounds as additives in rape seed oil for a steel-steel friction pair were investigated with two four-ball machines. The worn surfaces of the steel balls lubricated with the additive-containing rape seed oil were analysed by means of X-ray photoelectron spectrometry and scanning electron microscopy. Typical elemental distributions in the worn steel surfaces were analysed with an electron probe micro-analyser. It was found that rape seed oil containing thiophosphates as additives at appropriate concentrations demonstrated better tribological properties than tri-n-butyl phosphate. Tribochemical reactions were found to be involved for the steel-steel friction pair lubricated with the rape seed oil containing the synthetic phosphates,' with the formation of a boundary lubricating film composed of a glyceride of the rape seed oil and the tribochemical reaction products of the additives. This is assumed to contribute to the improvement of the tribological properties of the rape seed oil base stock. The action mechanism of the additives is also discussed.