Enhanced tribological performance of nano-cutting fluids with soybean oil and hybrid h-BN/MWCNT additive nanoparticles utilizing the minimum quantity lubrication method

This study explores the use of soybean oil-based nanolubricants that incorporate hexagonal boron nitride (h-BN), multi-walled carbon nanotubes (MWCNTs), and their hybrid at a concentration of 0.15 wt% for minimum quantity lubrication (MQL) in the computer numerical control (CNC) milling of AISI 1045 steel. Nano-cutting fluids were prepared via a two-step process, involving stirring followed by ultrasonication, and were characterized for density, viscosity, thermal conductivity, and rheology. Machining responses comprised tool wear length, cutting temperature, surface roughness, chip morphology, and chip colors. All samples exhibited Newtonian behavior at 40 °C and 100 °C. The hybrid h-BN/MWCNT formulation delivered the best overall performance, achieving the lowest tool wear length (~0.067 mm), reduced cutting temperatures, and the smoothest surfaces relative to dry cutting and single-additive fluids. The improvements are attributed to a synergistic mechanism in which h-BN platelets form a boundary-lubrication tribofilm, while MWCNT act as nano-bearings and a thermally conductive network that stabilizes the cutting zone. Simple sedimentation observations indicated higher dispersion stability for MWCNT-containing nano-cutting fluids than for h-BN alone. These results suggest that hybrid nano-additives in plant-oil-based cutting fluids may offer practical benefits for MQL-assisted machining or, particularly by helping reduce tool wear, improve thermal management, and support more sustainable machining.