The tribological behaviour and mechanism of action of sulphur‐containing borate esters in rape seed oil

Abstract

AbstractThree sulphur‐containing borate esters and one borate ester based on boric acid and n‐octanol were synthesised. The antiwear and friction‐reducing properties of these synthetic compounds as additives in rape seed oil were examined using a four‐ball machine. The relationship between the additive structure and tribological properties was explored, and the lubrication mechanisms of the additives investigated. The worn surfaces of a steel ball lubricated with the additive‐containing rape seed oil were analysed using X‐ray photoelectron spectroscopy and scanning electron microscopy. The elemental compositions of, and distributions in, the worn steel surfaces were determined with an electron probe micro‐analyser. It was found that sulphur‐containing borate esters as additives in rape seed oil at appropriate concentrations can be effective in improving antiwear and friction‐reducing behaviour. The rape seed oil base stock experiences competitive adsorption and tribochemical reactions with additives on the rubbing steel surface. The introduction of the active element sulphur into the borate ester additives functions to improve the tribological behaviour of the base stock by increasing the rate of formation of a protective layer on the rubbing steel surface. However, the tribological behaviour of the additives is not dependent on the number of sulphur atoms in the additive molecules, and further work is needed to investigate the possible dependence of this behaviour on the ratio of boron to sulphur in the additive molecules.