Superlubricity from mechanochemically activated aromatic molecules of natural origin: A new concept for green lubrication

As much as 23 % of the world's energy is consumed in tribological contacts and reducing friction and wear can have a substantial environmental impact. However, the large majority of lubricants and additives, still based on crude oil, are becoming less and less sustainable: environmentally friendly alternatives are in dire need. Hypericin, a natural constituent of St. John's wort commonly known as antidepressant/antiviral/antibiotic medicinal, is here revealed to have an outstanding potential as friction modifier. Indeed, sustainable superlubricity performance, with friction coefficient below 0.01 under boundary lubrication, is observed when adding hypericin to glycerol to lubricate steel/SiC tribopairs. XPS and Raman spectroscopies revealed the formation of graphitic structures on surfaces rubbed in the presence of hypericin and High-Resolution-Transmission-Electron microscopy on focused ion-beam cross-sections evidenced the first stage of tribofilm polymerization and subsequent formation of graphene. First-principles calculations elucidate the thermodynamic forces driving graphene-formation induced by mechanochemical reactions. The process, tribologically promoted, was monitored in real-time by (ab-initio) molecular dynamics. Formation of graphene patches was found to strongly correlate with friction coefficient reductions up to the superlubricity limit. This work suggests an unconventional way towards formation of graphene-like materials through mechanochemistry and reveals the great potential of pharmacopeia-derived molecules as newly-emerging environment-friendly lubricant additives for industrial applications, opening new ways to formulate water-based lubricants, nowadays considered a strategic oilsalternative to reduce carbon footprint.