Tribological improvement of sunflower lubricants using different surface modified graphene additives

The use of mineral lubricants has been demonstrated to be detrimental to the environment. Consequently, there has been a shift toward the utilization of biodegradable lubricants, such as those derived from vegetable sources. The last ones necessitate the incorporation of additives to attain optimal anti-friction and anti-wear characteristics, with nanoparticles emerging as particularly suitable candidates due to their exceptional properties. However, they have the significant limitation that they sediment in the fluid, making their chemical modification to achieve good stability a challenge. Thus, the objective of this work is analyzing the antifriction and antiwear properties of sunflower nanolubricants that contain sunflower oil and graphene oxide (GO), alkylated hexadecanol graphene oxide (GO-HD) and alkylated sulfur-doped graphene oxide (GO-SA) nanoparticles. Slight enhancements in friction were observed compared to sunflower oil, with a maximum 11 % reduction for GO-HD (0.10 wt%). With regard to anti-wear behavior, GO-HD and GO-SA nanolubricants exhibited outstanding performance, with maximum worn volume reductions of 44 % and 36 %, respectively. Finally, through Raman spectroscopy and scanning electron microscopy the tribofilm formation as tribological mechanism responsible for tribological improvement was observed. • GO-HD and GO-SA nanoparticles were synthesized from GO and fully characterized. • GO-HD and GO-SA sunflower nanolubricants show better stability than GO ones. • 0.10 wt% GO-HD was the optimal friction performance (11 % reduction). • 0.10 wt% GO-HD was the optimal antiwear behavior (44 % worn volume reduction). • Lubrication mechanism can be explained by tribofilm formation.