Formation of robust graphene tribofilms on hydrogen-free amorphous carbon/boron carbide periodic nanomultilayer coatings achieving low friction

Achieving superlubricity in hydrogen-free amorphous carbon (a-C) coatings through the formation of multilayer graphene-based tribofilms via tribochemical reactions has remained a significant challenge. Here, we report the successful fabrication of H-free a-C/B₄C periodic nanomultilayer coatings (PNCs) with systematically varied modulation periods (3–8.5 nm), which achieve low friction through tribochemically induced graphene tribofilm formation. The optimal coating structure, consisting of alternating 1.5 nm a-C and 2.5 nm B₄C nanolayers (a-C1.5), demonstrated a remarkably low coefficient of friction of ~0.075 and exceptional wear resistance (~1.3×10⁻¹⁰ mm³/(N·mm)) under ambient conditions. Advanced characterization techniques, including 2D Raman mapping and high-resolution transmission electron microscopy (HRTEM), confirmed the formation of multilayer graphene structures on the counter surface and the wear track during sliding. The formation of this multilayer graphene-based tribofilm, previously reported only in hydrogenated a-C coatings, transforms asperity-dominated shearing into low-resistance interlayer sliding between graphene nanoflakes. After extending the reciprocating sliding to 60k cycles, the tribofilm persisted, even though the coating had worn out. Molecular dynamics simulations revealed that the superior tribological performance originates from enhanced elastic recovery, optimal stress distribution across nanolayers, and boron-promoted tribochemical graphitization of amorphous carbon. This discovery demonstrates that graphene-based tribofilms can be derived from H-free a-C-based coatings, opening a novel and practical strategy for achieving low friction in carbon-based protective coatings without the limitations associated with hydrogen content or preconstructed 2D material additives.