Study of the tribological behavior of sulfurized fatty acids as additives in rapeseed oil

Odorless sulfurized fatty acids, sulfurized octadecanoic acid (SOA) and sulfurized docosanoic acid (SDA), were synthesized. The tribological behavior of the synthesized compounds as additives in rapeseed oil was evaluated and compared with octadecanoic acid using four-ball friction and wear testers. The morphologies and elemental chemical states on the worn surfaces of the lubricated GCr15 steel were investigated by means of X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). As the results, both SOA and SDA as additives in rapeseed oil show improved extreme pressure properties and friction-reducing behavior as compared with the rapeseed oil alone. They show better antiwear and extreme pressure properties than octadecanoic acid additive at relatively higher load. The differences in the tribological behavior of octadecanoic acid and SOA and SDA are attributed to their different action mechanisms therein. That is, octadecanoic acid functions by forming adsorption film alone while the two sulfurized fatty acids function by forming adsorption film and tribochemical reaction film during the friction process.