<i>N</i> -Alkylamino-Functionalized Multiwalled Carbon Nanotubes for Advanced Grease Applications: Stability and Tribological Enhancement

Advanced lubricants play a critical role in modern industry by reducing friction, minimizing wear, and extending the service life of heavily loaded mechanical systems. Addressing these challenges requires the development of nanocomponents capable of simultaneously improving multiple tribological parameters without compromising the consistency or manufacturability. This study investigates the potential of <i>N</i>-alkylamino functionalized multiwalled carbon nanotubes (MWCNTs) as high-performance nanoadditives for industrial grease formulations. The aim is to evaluate their impact on the dispersion stability and key tribological properties under severe operating conditions. <i>N</i>-alkylamino MWCNTs were synthesized via one-step nucleophilic addition of terminal <i>n</i>-alkylamines with chain lengths of C₆, C₁₂, and C₁₆ to pristine MWCNTs yielding the functionalization degrees equal to 3.2, 2.8, and 10.3 wt %, respectively. This approach ensured atom economy, eliminated side products and problematic wastes, and operated under relatively mild conditions with high overall efficiency. The resulting nanoadditives were dispersed in a base mineral oil and incorporated at as low as 0.02 wt % into lithium-based greases containing nanographite and MoS₂ as the typical commercial additives. The functionalized MWCNTs demonstrated excellent time and operational stability in the base oil, with no observable sedimentation over 7 days. Greases formulated with <i>N</i>-alkylamino-functionalized MWCNTs demonstrated markedly superior tribological performance, achieving a weld load of up to 490 kG (versus 250 kG for the commercial reference), reduced wear scar diameters of 0.49-0.50 mm (approximately 11% lower), and an exceptionally low SRV friction coefficient of 0.1030, representing a reduction of up to 84% compared to the commercial grease. Notably, these enhancements were achieved while maintaining an NLGI grease consistency class of 2, ensuring practical applicability and processability. This approach offers a scalable route to next-generation lubricants suitable for highly loaded tribo-pairs, precision mechanical assemblies, and energy-efficient drivetrains.