Application of GNP and MWCNT-based Nano-MQL lubrication combined with vortex cooling when machining of bearing steel

This study investigates the performance of environmentally friendly hybrid cooling/lubrication strategies for the machining of 100Cr6 bearing steel, with a particular focus on integrating nanofluid-based minimum quantity lubrication and vortex tube cooling. Eight different cooling/lubrication methods were used: i.e., dry, MQL, Vortex, MQL+Vortex, MWCNT, MWCNT+Vortex, GNP, and GNP+Vortex. To characterize the tribological and thermophysical behavior of the nanofluids, viscosity, pH, and thermal conductivity measurements were conducted. Machining performance was assessed in terms of cutting temperature, surface roughness, power consumption, and flank wear under controlled turning experiments. The experimental results clearly demonstrate that hybrid systems outperform both conventional and single-mode cooling strategies. Among the tested conditions, the MWCNT + vortex configuration showed the greatest improvements across all performance indicators. Compared to dry machining, this hybrid approach reduced cutting temperature by 49.85%, surface roughness by 22.89%, power consumption by 20.97%, and flank wear by 87.94%. Similar, though slightly lower, improvements were observed in GNP-based hybrid conditions, confirming the effectiveness of nanoparticle-assisted lubrication in enhancing tribological performance. Overall, the combination of vortex tube cooling and nanofluid-assisted MQL creates a highly effective hybrid strategy for improving the machinability of 100Cr6 bearing steel. The results clearly indicate that MWCNT-based nanofluids combined with vortex cooling provide the most favorable balance between thermal management, energy efficiency, and tool protection.