Vegetable oil-in-water emulsions are potential metal-working lubricants. These emulsions are susceptible to oil-water phase separation due to the oil droplets' coalescence. In this study, we enhance the dispersion stability and lubricity of such emulsions with TiSiO4 nanoparticles (NPs). With 1 vol% soybean oil, stable emulsions are obtained with 0.25 to 0.75 wt% NPs, attributed to formation of shielding layers of NPs around the droplets. Lowest friction and wear are obtained without and with 0.25 wt% NPs, respectively. With increasing the NPs' mass fraction, NP-shielded droplets become less attracted to metal surfaces. Association of NPs within boundary film is also found when NPs' mass fraction is above 0.5 wt%, inducing third body abrasive behaviour. Further increasing the NPs' mass fraction results in the formation of agglomerates of NPs in emulsions which increases vibration, friction, and wear. Findings suggest that a mass fraction of NPs exists at which stable emulsions with improved lubrication performance are obtained.
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