High-Load-Triggered Nanoscale Superlubricity in Long-Chain Borate Ionic Liquids

Achieving nanoscale superlubricity with ionic liquids (ILs) in the absence of external fields or additives remains a major challenge in tribology. Here, we investigate the nanoscale friction of a homologous series of three borate-based ILs (denoted A4, A8, and A12) with a varying anion alkyl-chain length [A4BMB]−, [A8BMB]−, [A12BMB]−, and a common cation trioctyldodecylammonium ([N88812] +) on graphite surfaces, using AFM, in situ AFM-IR, angle-resolved XPS (AR-XPS), and nonequilibrium molecular dynamics (NEMD) simulations. Both A4 and A8 show stable superlow friction across the load range (μ ≈ 0.0032 and 0.0068, respectively). A12, however, demonstrates a frictional transition triggered by an increasing load. The friction μ ≈ 0.023, obtained at a low load, drops drastically to μ ≈ 0.0013 once the normal load exceeds ∼30 nN (≈2.4 GPa) and enters a clear superlubric state. As neither AFM-IR nor AR-XPS reveals any tribochemical transformation during this transition, it appears to be a purely physical, load-induced structural reorganization of the interfacial ion layers. Further studies show near-homogeneous cation/anion distributions in A12 at the interface, while NEMD simulations can identify load-dependent reorientation of the long anion alkyl chains that reduces interfacial locking and shear resistance. Our results show a purely mechanical unlocking pathway to nanoscale superlubricity in ILs and suggest that long-chain borate anions can be used for extreme-pressure lubrication in micro/nano electromechanical systems.