Tribochemistry of mercaptoimidazoline as an additive in emulsion between self-mated GCr15 ball interfaces and its friction-reduction mechanism

Abstract The tribological properties of two mercaptoimidazolines, with amino (2-MBTA) and hydroxyl (2-MBTD) functional groups, as lubricating additives in a water-based rapeseed oil emulsion were investigated. The results of tribological tests on a four-ball tester showed that both mercaptoimidazolines had effective anti-wear and friction reduction properties. The optimal tribological performance of the 2-MBTD was found to be 98 N and could be scribed to the hydroxyl group, whereas the amino group in 2-MBTA created a more effective wear-reducing tribofilm on the rubbing surfaces at 294 N. EDS analysis on the worn surface areas showed that the difference in tribological properties between 2-MBTA and 2-MBTD was attributed to the different contents of the active elements N and S. In addition, XPS results indicated that sulfates were prevalent in the tribofilm for 2-MBTA, whereas sulfides that could reduce the coefficient of friction (COF) were present in the tribofilm for 2-MBTD. The friction mechanism of 2-MBTA and 2-MBTD is that different functional groups influence the composition of the boundary lubrication film formed, which in turn affects the tribological properties of the additives.