Surface-engineered hexagonal boron nitride via RAFT-synthesized methacrylates for enhanced dispersibility and tribological performance in castor oil

Hexagonal boron nitride (h-BN) is a promising solid lubricant and oil additive due to its high thermal stability and chemical inertness; however, its poor dispersibility in non-polar media limits practical use. In this study, a monomer-level hydrophobic design strategy was employed to overcome this issue. Methacrylate monomers were synthesized by Steglich esterification of 2-hydroxyethyl methacrylate (HEMA) with pentanoic, lauric, or palmitic acid, followed by RAFT polymerization using a carboxyl-functional chain transfer agent. The resulting homopolymers were covalently grafted onto hydroxylated h-BN nanosheets to yield hybrid nanostructures with tailored hydrophobic side chains. Structural analysis confirmed the formation of well-defined polymers and preservation of h-BN's layered structure after grafting. Dispersion tests in castor oil over 21 days revealed that short-chain polymers enhanced stability, while long chains led to sedimentation due to hydrophobic aggregation. Tribological performance was evaluated under boundary lubrication using a ball-on-disc tribometer. Friction coefficients, wear rates, and wear morphologies were analyzed. Notably, h-BN grafted with poly(lauryl methacrylate) (PLAMA) achieved the lowest steady-state friction coefficient (0.0646) and wear rate (0.01132 mm³/N·m), corresponding to 59 % reduction in friction and 86 % in wear compared to neat castor oil. These improvements are attributed to improved oil compatibility, interfacial adhesion, and tribofilm formation. This study demonstrates how polymer design governs dispersion and tribological behaviour, offering a scalable approach to engineering oil-compatible 2D lubricant additives.