Next-gen hybrid greases: Ionic liquids and nickel-functionalised carbon nanotubes in metal-polymer lubrication

This study explores a new approach to hybrid lubrication by combining an ionic liquid with nickel-decorated carbon nanotubes. The aim was to develop a grease capable of merging the chemical activity of the ionic liquid with the tribochemical adaptability of the nanocarbon phase. TEM/EDX analyses confirmed an efficiency of the nickel decoration, resulting in locally wrapped nanotube clusters that may influence interfacial reactivity. Friction tests performed for polyamide-6 (PA) and ultra-high molecular weight polyethylene (UHMWPE) sliding, as polar and non-polar polymers, respectively, against AISI 4130 steel showed a marked reduction in friction for all ionic-liquid-based lubricants compared with a commercial reference polyurea grease. For PA, the synergy between the ionic liquid and Ni-CNTs produced the lowest and most stable friction, attributed to the exfoliation of CNTs into quasi-graphene lamellae that formed shear-adaptive tribofilms. In UHMWPE, the neat ionic liquid provided the best frictional response, as CNTs remained embedded within the transfer layer and did not participate in lamellar film formation. Overall, the results demonstrate that ionic-liquid- and nanocarbon-based hybrid greases can be tuned to promote either chemical or lamellar lubrication pathways, depending on the mechanical response of the polymer. These findings indicate that the tribological response of the proposed hybrid grease is governed by boundary-film evolution at the polymer-metal interface rather than by a universal nano-additive effect. In this respect, IL + Ni-CNTs favoured the formation of a shear-adaptive lamellar tribofilm in PA, whereas in UHMWPE the ionic liquid remained the dominant friction-reducing phase and CNTs acted mainly as transfer-layer-reinforcing anti-wear fillers.