Laboratory evaluation of tribological performance of biodegradable grease with magnetite nanoparticles coated with stearic acid for wheel/rail interface

A biodegradable grease with the addition of magnetite nanoparticles coated with stearic acid (MagNP) was evaluated for its lubricity in a pure sliding sphere-on-disc test, representing the contact between the wheel flange and the rail gauge face on curves. The tests were conducted to meet the parameters typically found in tropical heavy-haul railways. Its performance was compared with eleven other greases: five commercials, one being the base grease to which the MagNPs were added, and six formulated in-house. The presence of 0.5% MagNP resulted in a coefficient of friction (CoF) reduction of 36.4% and a total wear volume reduction of 67.4% compared to the base grease. This grease also exhibited the lowest CoF and wear among the twelve tested greases, demonstrating that the addition of MagNP not only improved upon its corresponding base grease but also yielded better lubricity results than the combination of other oils, thickeners, and additives currently used in heavy-haul tropical railway greases. Worn surface evaluation indicated that the MagNP protected the surface against debris, resulting in wear profiles on the sphere and disc that differed from those produced by the other eleven greases evaluated. Additionally, the MagNP attracted more additives present in the grease to the surface and acted by polishing the surface, one of the mechanisms of action of nanoparticles as additives. MagNP shows potential for the development of biodegradable greases with good performance for railway applications.