Optimization and Performance Evaluation of a Bio-Based Lubricating Grease from Virgin Coconut Oil, Beeswax (Apis cerana), and Alumina Nanoparticles (Al₂O₃) Using a D-Optimal Mixture Design.

A bio-based lubricating grease was formulated from virgin coconut oil (VCO) as the base oil, Apis cerana beeswax as the thickener, and alumina (Al₂O₃) nanoparticles as the additive, with component proportions constrained to 0–5 wt% Al₂O₃, 20–30 wt% beeswax, and 70–80 wt% VCO. Twelve formulations were generated through a D-Optimal Mixture Design in Design-Expert® and characterized for dropping point and apparent viscosity. All formulations produced stable, semi-solid greases with no observed phase separation, with dropping points ranging from 55°C to 59°C and apparent viscosities ranging from 0.0538 Pa·s to 3.4986 Pa·s. Statistical modeling showed that beeswax and VCO significantly affected dropping point, while Al₂O₃ exerted its influence on apparent viscosity primarily through binary and ternary interactions with the other components. Numerical optimization using the desirability function identified 0.00 wt% Al₂O₃, 30.00 wt% beeswax, and 70.00 wt% VCO as the optimal formulation, yielding a predicted dropping point of 58.357°C and apparent viscosity of 0.204 Pa·s, with an overall desirability of 0.839. This formulation was re-prepared and subjected to a Rotating Disk Electrode (RDE) Fine Wear Test under actual gear contact conditions, where iron emerged as the dominant wear metal at 179.7 ppm, consistent with the steel composition of the test gears, while secondary wear metals remained at comparatively low concentrations. These results indicate that a fully bio-based grease composed of VCO and beeswax alone, without nanoparticle reinforcement, can achieve a statistically optimized balance of thermal stability and rheological performance while exhibiting preliminary evidence of adequate wear protection under real contact conditions.