Tribological synergies among chemical-modified graphene oxide nanomaterials and a phosphonium ionic liquid as additives of a biolubricant

In the present work, antifriction and antiwear synergies of two functionalized graphene oxides (reduced graphene oxide, rGO, and reduced graphene oxide modified with octadecylamine, rGO@ODA) with a phosphonium ionic liquid (IL) as additives of a biodegradable ester base oil, BIOE, were investigated. For this aim, four BIOE nanodispersions were formulated: two nanodispersions without IL and two others with IL (hybrid nanolubricants), being the nanodispersions: BIOE + 0.05 wt% rGO, BIOE + 0.05 wt% rGO@ODA, BIOE + 1 wt% IL + 0.05 wt% rGO and BIOE + 1 wt% IL + 0.05 wt% rGO@ODA, and all of them showing good temporal stability (at least 3 weeks), especially those containing the ionic liquid. Tribological tests, under pure sliding conditions, were made with the aforementioned nanolubricants and with neat BIOE under a 20 N working load. All hybrid and non-hybrid nanolubricants resulted in lower friction coefficients compared to BIOE oil, with the largest friction reduction of 34% achieved by the 1 wt% IL + 0.05 wt% rGO nanolubricant. Discs lubricated with the prepared nanolubricants showed less wear than discs lubricated with BIOE, achieving the highest reduction (34%) in wear track width also for the 1 wt% IL + 0.05 wt% rGO nanolubricant. In addition, through confocal Raman microscopy, as well as roughness analysis of tested discs, it can be concluded that surface repairing, synergistic effect and tribofilm formation mechanisms occur. (C) 2021 Elsevier B.V. All rights reserved.