Tribological performance and tribofilm formation of hBN-enhanced glycerol-based lubricants

The development of sustainable lubricants that effectively reduce friction and wear remains challenging, particularly under boundary and mixed lubrication conditions. Incorporating solid lubricant additives is a promising approach to improve the tribological performance of biodegradable lubricants. Among these additives, hexagonal boron nitride (hBN) has attracted considerable attention because of its layered structure and low shear strength. Nevertheless, its tribological behaviour and tribochemical interactions in glycerol-based lubricants remain insufficiently understood. In this work, pure glycerol and aqueous glycerol containing 0.02 and 0.1 wt% hBN were evaluated in steel/steel contacts using a reciprocating ball-on-disc tribometer under normal loads of 3 and 40 N. Among the investigated formulations, 0.1 wt% hBN provided the best tribological performance. Under a load of 40 N, the addition of 0.1 wt% hBN reduced the steady-state coefficient of friction from ~0.10 to ~0.05 in pure glycerol and from ~0.09 to ~0.04 in aqueous glycerol. The effect on wear morphology depended on the base fluid, with the maximum penetration depth decreasing in the glycerol system but increasing in the aqueous glycerol system, despite a reduction in wear-track width. FTIR, Raman, and XPS analyses revealed tribochemical interactions and the formation of a protective hybrid tribofilm. The hBN-derived material present within the tribofilm underwent less pronounced structural modification in pure glycerol than in aqueous glycerol. These findings demonstrate the potential of hBN-modified glycerol lubricants for environmentally friendly boundary and mixed lubrication applications.