Structural characterisation of iron sulfide tribofilms formed on carbon and stainless steels

This study investigated tribofilm layers formed under boundary lubrication using a polysulfide additive on S45C and SUS304 steel surfaces. In the tribotests, the friction coefficient of the SUS304 disc fluctuated and increased more gradually in comparison to that of the S45C disc. The tribofilm on the S45C disc comprised the outermost and inner layers of FeS₂ (pyrite) and FeS (troilite), respectively. Notably, all the tribofilm layers on the SUS304 disc surface contained pyrite. The tribofilm on the SUS304 disc was weakly crystalline in the outermost layer and thinner than that on the S45C disc. The poor formation of the tribofilm on the SUS304 disc was potentially caused by the unstable friction coefficient and scuffing.