REVIEW ON PERFORMANCE EVALUATION OF CASTOR OIL EMULSIONS FOR SUSTAINABLE MINIMUM QUANTITY LUBRICATION (MQL) IN DRILLING

This comprehensive review has synthesised the findings of 45 peer-reviewed experimental studies on the performance of castor oil emulsions for MQL-assisted drilling published between 2010 and 2024. The following principal conclusions are drawn:   •       Castor oil's unique molecular structure — characterised by the C-12 hydroxyl group on ricinoleic acid (85–92% composition) — provides exceptional boundary lubrication through strong polar adsorption onto metallic surfaces, making it the most effective single-component vegetable oil for MQL drilling applications. •       Across all workpiece materials reviewed, castor oil MQL achieves cutting temperature reductions of 18–35% relative to dry drilling and reduces surface roughness (Ra) by 20–51%, with the greatest benefits observed in difficult-to-cut materials (Ti-6Al-4V, Inconel 718, AISI 316L) where thermal effects and adhesion are dominant machining challenges. •       Optimal MQL parameters for castor oil emulsions are consistently identified in the ranges: concentration 10–15 vol.%, air pressure 4–6 bar, flow rate 80–150 mL/h, and nozzle angle 30–45° to the drill axis — with specific optima varying by workpiece material and cutting conditions. •       Tool life under castor oil MQL exceeds dry machining by 30–65% and in several reported studies also surpasses conventional flood cooling by 15–35%, owing to the formation of protective tribofilms from ricinoleic acid and its thermally degraded products. •       Nano-enhanced castor oil emulsions incorporating Al₂O₃, MoS₂, or graphene nanoplatelets at optimal concentrations (0.1–0.5 wt.%) provide incremental performance improvements of 10–33% over plain castor oil MQL in surface roughness and tool wear, with synergistic effects observed in hybrid nanoparticle systems. •       Life cycle assessment confirms that castor oil MQL reduces global warming potential by up to 71% and human toxicity potential by up to 84% compared to flood cooling, and achieves carbon footprint reductions of ~78% per unit of drilled holes, reinforcing its suitability as a sustainable machining strategy.

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