Load-Dependent Tribological Performance of a Rice Husk–Derived Lithium Grease Without Conventional Extreme Pressure Additives

Abstract

This study evaluates the load-dependent tribological performance of a lithium grease formulated from rice husk–derived materials, comprising rice husk pyrolysis oil (60 wt%), lithium stearate thickener (35 wt%), and rice husk ash additive (5 wt%). Gravimetric wear tests were conducted at 1490 rpm under applied loads of 800 g, 900 g, and 1000 g, and the results were benchmarked against two commercial lithium-based greases. At 800 g, all greases exhibited comparable specific wear rates on the order of 10⁻⁶ – 10⁻⁷ g·N⁻¹·s⁻¹. However, at 1000 g, both commercial greases failed prematurely, entering a severe wear regime with specific wear rates of approximately 3.4–3.6 × 10⁻⁴ g·N⁻¹·s⁻¹. In contrast, the rice husk–derived grease completed the full 300 s test duration with a significantly lower wear rate of 1.70 × 10⁻⁶ g·N⁻¹·s⁻¹, corresponding to about a 99.5% reduction relative to the commercial formulations. This superior load-bearing performance is attributed to synergistic effects between polar oxygenated compounds in the bio-oil, the mechanically stable lithium stearate network, and load redistribution by finely dispersed rice husk ash particles. Notably, this performance was achieved without the use of conventional sulfur- or phosphorus-based extreme-pressure additives, demonstrating the potential of agro-residue-derived greases as a sustainable and high-performance alternative for lubrication applications.