Experimental Characterization of Oleic Acid–Stabilized SiO2 Nanolubricants for Enhanced Viscosity, Thermal, and Lubrication Performance of VG 32 Oil

As industrial machinery demands higher efficiency and reduced energy losses, nanolubricants (NLs) have emerged as promising next-generation tribological fluids for rotor bearing systems. The present study has formulated and evaluated SiO 2 -based NLs using oleic acid (OA) as a surfactant in VG-32 mineral oil, focusing on dispersion stability, rheological, thermal, and lubricity performance. SiO 2 nanoparticles (NPs) were incorporated at concentrations of 0.05–0.75 weight % (wt%) via a two-step process with OA at a 1:3 ratio. Sedimentation tests and zeta potential measurements confirmed good electrostatic stability, with values within ±30 mV. Temperature-dependent viscosity measurements revealed improved viscosity index and thermal-viscosity stability, with 1:3 OA surfactant coated formulations demonstrating superior performance over uncoated NLs. Thermal conductivity showed a progressive enhancement with SiO 2 NPs concentration, reaching a maximum of 8.7% at 0.75 wt% (0.135 W/m·K) compared to base oil (0.121 W/m·K). Flash and fire point measurements confirmed enhanced thermal safety, with the 0.4 wt% formulation achieving the highest fire point of 226°C, a 6°C improvement over the base oil. Tribological evaluation using a block-on-ring tribometer at a constant load of 100 lbf and speed of 60 rpm revealed that the 0.4 wt% SiO 2 NLs with 1:3 OA ratio achieved the lowest coefficient of friction (COF), representing ∼20% reduction over base oil, which is attributed to ball-bearing and mending effects at the contact interface. The coupled influence of zeta potential and NPs loading establishes a strong stability-performance relationship, supporting the development of energy-efficient nanolubricants for adjustable journal bearing systems.