Scuffing Resistance of Castor Oil and Aminolyzed Castor Oil Enhanced with MoS₂ Nanoparticles

Abstract The transition toward sustainable industrial practices has led to the development of eco-friendly vegetable oils (biolubricants) as alternatives to traditional petroleum-based lubricants. However, vegetable oils are inherently susceptible to oxidative and thermal degradation at high temperatures. To overcome this, chemical modification of the pure oil is commonly employed, which not only enhances thermo-oxidative stability but also improves intrinsic lubricity. To further boost lubrication under severe conditions, dispersing solid nanoparticles into the oil has emerged as a highly effective strategy. Therefore, this study investigates the scuffing behavior of MoS₂-based nanofluids formulated with castor oil (CO) and a chemically modified vegetable oil (aminolyzed castor oil—ACO). Scuffing tests were performed in a block-on-ring tribometer. Nanofluids containing 0.10, 0.15, and 0.20 wt.% MoS₂ were compared to their additive-free base oils. Friction force evolution was quantitatively analyzed to identify characteristic scuffing phases. Results demonstrated that MoS₂ increases severe scuffing resistance, with this resistance strongly depending on the base oil and nanoparticle concentration. Among the formulations, ACO with 0.10 wt.% MoS₂ exhibited the best performance, increasing the Weibull characteristic life (α) by approximately 36.7% compared with additive-free ACO. Furthermore, the nanofluids exhibited a more stable friction response during severe scuffing, in contrast to the pronounced friction instabilities observed with the additive-free base oils. SEM/EDS analyses revealed molybdenum oxides on worn surfaces, indicating tribochemical reactions and protective tribofilm formation activated by high temperature. Overall, chemical modification of the base oil (i.e., ACO) enhances the beneficial effects of MoS₂ nanoparticles, highlighting ACO-based nanofluids as promising lubricants for severe tribological applications.