Toward lithium-free tribology: Design and performance of plant oil-based greases thickened with functionalized nanoclays

To address the persistent environmental concerns of petroleum-based lubricants, we report on a new class of high-performance, sustainable greases. These were formulated by combining select vegetable oils, including the structurally unique Orychophragmus violaceus (OV) seed oil, with an oleic acid-functionalized organo-modified montmorillonite nanoclay thickener. The sustainable samples consistently outperformed an industrial-grade lithium soap grease in tribological experiments. Remarkably, under harsh conditions of high load (up to 500 N) and temperature (150 °C), both 5 and 10 wt% nanoclay-thickened OV-based greases exhibited superlubricity, with the coefficient of friction (COF) of approximately 0.008. Surface analysis using Raman spectroscopy after the tribological experiments revealed that this exceptional performance was not a result of typical carbon-based tribofilm formation but from a novel lubrication mechanism: the in-situ formation of an adhesive, low-shear solid tribofilm composed of the nanoclay itself. The formulated greases also exhibited excellent low-temperature fluidity and structural stability, with complete recovery after freeze-thaw cycles to −20 °C. These results highlight a new design methodology for creating reliable and biodegradable lubricants suitable for extreme industrial applications.