Tribological characteristics of di(iso-butyl) polysulfide as extreme pressure additive in some mineral base oils

Purpose - In order to develop novel high EP S-containing additives and to meet the need of formulating GL-5 gear oil or other high EP lubricating oils, aims to investigate the tribological behaviors and mechanism of a di(iso-butyl)polysulfide (DIBPS), which was synthesized from some cheap materials at low temperature and under normal atmospheric pressure, as an additive in some mineral base oils compared with the traditional sulfurized olefin (SO) additive. Design/methodology/approach - The DIBPS additive was designed and synthesized, of which the main composition is the di(iso-butyl)trisulfide. Its load-carrying capacity, anti-wear and friction reduction properties as additive in some mineral base oils, compared with the traditional SO additive, were investigated using a four-ball machine and a Timken tester according to relative testing standards. The tribological mechanism was discussed according to the SEM and XPS analytical data. Findings - The results indicate that the four-ball P_D value and the Timken OK value of the prepared DIBPS in VHVIS500 is clearly better than that of the traditional SO; the anti-wear property of DIBPS is equivalent to the traditional SO and the friction reduction effect of DIBPS is better than that of the traditional SO. The SEM and XPS data show that the DIBPS additive experiences different tribochemical reaction during tribological process compared with the traditional SO. The S active element of DIBPS reacted with surface metal mainly to form FeSO₄ and/or Fe₂(SO₄)₃ inorganic film, but the S active element of the traditional SO reacted with the surface metal mainly to form FeS inorganic film. This may be the chief reason why the prepared DIBPS possesses better EP properties than the SO. Research limitations/implications - The results show that the polysulfide additive (DIBPS) possesses better extreme pressure property than the traditional SO. However, more experimental study such as the synergic effect with other additives must be performed, from which it will be clearly shown whether the novel polysulfide can be applied in industrial oils. Practical implications - These results may be useful for the researchers to formulate some high EP industrial oils. Originality/value - This paper proves that the designed polysulfide additive, of which the main composition is the trisulfide, possesses better extreme pressure property than the traditional SO, and its tribological mechanicsm is also different with that of SO. It is noticed that the preparative method of this novel polysulfide additive has some superiorities, such as: low-experimental temperature, low-experimental pressure and cheap materials.