Surfactant-free CuO nanoball-based nanolubricants: Experimental insights into dispersion stability, thermo-rheological properties, and tribological behavior

This study presents a comprehensive investigation into the synthesis, dispersion behavior, and performance evaluation of surfactant-free copper oxide (CuO) nanoballs (NBs) dispersed in polyalphaolefin (PAO) oil. CuO NBs were synthesized via a modified precipitation technique and characterized via X-ray diffraction (XRD), Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), and transmission electron microscopy (TEM), confirming their monoclinic crystal structure and spherical morphology with particle sizes ranging from 25 to 132 nm. The dispersion quality and long-term stability of the nanolubricants were assessed via UV–Vis spectroscopy and zeta potential analysis, which indicated that 0.01 wt% CuO achieved the highest stability (zeta potential: 154.3 mV) and minimal sedimentation for up to 10 days. Rheological measurements revealed Newtonian behavior across all concentrations, with the highest relative viscosity observed at 0.05 wt% and 100 °C. The viscosity index improved at lower concentrations, supporting the lubricant’s thermal adaptability under dynamic shear conditions. The thermal conductivity increased with CuO addition, peaking at 0.01 wt%, primarily due to enhanced Brownian motion and reduced nanoparticle agglomeration. Tribological performance, evaluated using a reciprocating tribometer under a 10 N load and 840 m stroke length, revealed that 0.01 wt% CuO achieved a 37% reduction in the coefficient of friction (COF 0.055) and the lowest specific wear rate among all the tested samples. Surface analysis via three-dimensional (3D) profilometry and SEM-energy dispersive X-ray spectroscopy (EDS) revealed smoother contact surfaces and no evidence of CuO deposition, suggesting that a rolling friction mechanism was the dominant lubrication mode. These findings confirm that surfactant-free CuO NBs significantly enhance the tribological, rheological, and thermal properties of PAO oil, offering a cost-effective and environmentally friendly solution for high-performance industrial lubrication systems.