Hypericin enhances superlubricity of glycerol by acting as graphene precursor

Hypericin (C₃₀H₁₆O₈) is a naturally occurring substance, an anthraquinone derived from St. John’s wort, possessing outstanding antiviral, antitumor, antibacterial, and antioxidant properties. Today, hypericin is primarily used in medicinal applications. It is a small, flat organic molecule with a graphene-like core surrounded by oxidized functions, suggesting it could act as a graphene precursor in tribological contacts. Therefore, we investigated the lubrication properties of hypericin as an additive in glycerol, used as a base oil. It is well established that glycerol is superlubricious under full and thin film elastohydrodynamic (EHD) lubrication regimes but generally fails with steel under more severe conditions (mixed and boundary regimes). We studied the effect of hypericin added to glycerol for steel-on-steel and steel-on-silicon friction pairs. For the steel-on-steel configuration, results show that hypericin is a strong anti-wear additive due to its antioxidant properties that scavenge OH radicals. Moreover, hypericin is also an efficient friction-reducing agent, providing a steady state and robust ultralow friction coefficient (0.02–0.03). Thus, it outperforms most traditional additive formulations under the same conditions, although it does not achieve superlubricity (coefficient of friction (CoF) < 0.01) under more severe conditions. For steel-on-silicon, hypericin significantly extends the superlubricity regime of glycerol to lambda ratios well below unity (low sliding speeds). The mechanism of superlubricity is attributed to the friction-induced formation of graphene layers from hypericin molecules, smoothing friction surfaces, and operating a hybrid liquid–solid superlubricious system.