Comprehensive Study on Gear Oil Modified with Lignin–Cellulose Nanofibers: Rheological and Tribological Response

The tribological, rheological, and acoustic performance of gear oil dispersed with lignin-cellulose nanofibers (LCNFs) are examined. Samples were performed with loading content of 0.25%, 0.5%, 0.75%, and 1.0%. The morphological and structural characterization of the LCNFs was confirmed using AFM, SEM, XRD, and FT-IR evaluations. The results revealed a substantial reduction in COF and wear rate at best loading content, with 0.5% by weight LCNFs, by up to 42% and 25.5%, respectively. Vibration and sound pressure level measurements showed significant damping behavior, with reductions of 18–25% at low and optimal loading level. Surface analyses of worn specimens (3D topography and SEM) confirmed the formation of a protective LCNF tribo-film at optimal dispersion, while excessive loadings above 0.5% by weight LCNFs resulted in abrasive features associated with particle clustering. In summary, LCNFs seem to be a very promising and fundamental ingredient in gear oil. The evolution of highly effective and sustainable nano-lubricants is highlighted by the potential to improve dynamic response, lower wear and friction, and increase viscosity stability.