In the present work, first, we show that sliding Nitinol 60 alloy against steel under castor oil lubrication exhibits a new case of superlubricity (coefficient of friction CoF ≪ 0.01). So far, CoF below 0.01 have never been achieved under boundary lubrication at high contact pressure and in presence of vegetable oil as a green lubricant. Next, it is demonstrated that superlubricity is controlled by tribochemical reactions, involving chemical degradation of castor oil and the formation of metal oxy-hydroxides. Finally, to explain these findings, we propose a novel superlubricity mechanism consisting of hexanoic acid molecules intercalated between nickel and iron oxy-hydroxide lamellar layers, a structure very similar to the one found in Fe-Ni batteries. We propose that superlubricity is achieved due to repulsive electrostatic forces acting between the intercalated metal oxy-hydroxide lamellar compounds. This system would be suitable for practical engineering applications in many fields including biotechnologies.
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