Tribological Performance Analysis of Castor–Black Cumin Hybrid Oil Doped with Boron Nitride in Cold Rolling

In this study, the tribological performance of an environmentally friendly, high-performance castor oil–black cumin oil blend was investigated by adding hexagonal boron nitride (h-BN) nanoparticles at varying concentrations during cold rolling. Castor oil served as the base oil, and h-BN nanoparticles were incorporated at concentrations of 0.05%, 0.125%, and 0.25 wt.% into the system, which was optimized with 10 mg/mL of black cumin oil. Performance was evaluated on steel specimens at 5% and 20% reduction ratios. The results demonstrated that, compared to undoped castor oil under dry conditions, the rolling force was reduced by approximately 18% and 9% at 5% and 20% reduction ratios, respectively. The best performance was achieved with the hybrid lubricant containing 0.25 wt% h-BN. Under these conditions, surface roughness parameters reached their lowest values at a 20% reduction ratio; specifically, reductions of approximately 23%, 17%, and 21% were observed in Ra, Rp, and Rv, respectively, compared to undoped castor oil. Furthermore, with the same formulation at a 5% reduction ratio, improvements of up to 17%, 21%, and 15% were achieved in the Sk, Spk, and Svk parameters, respectively. Optical and microscopic images revealed that h-BN nanoparticles formed a tribofilm that filled surface valleys and reduced contact between the roller and the workpiece.