NbN, water-based lubrication, ultra-low friction, ultra-low wear


Water-based lubrication has attracted wide attention as an oil-free lubrication method owing to its greener and cleaner lubrication means. However, due to operating in the water environment, most moving parts would inevitably suffer from abrasion, rusting, and aging problems. Developing a novel solid-water composite system with ultra-low friction and wear will open new possibilities for innovative lubrication material research and development. Here, we first revealed the water-based lubrication behavior of a high-hardness niobium nitride coating (NbN). In a three-phase contact environment (water, air, and NbN), oxidation and hydrolytic reactions of NbN result in the formation of "colloidal solutions", containing Nb2O5 colloidal particles between the tribo-pairs. Utilizing the double electric layer repulsion and weak shear action of the "colloidal solution", NbN achieves ultra-low friction and wear; the corresponding values are as low as 0.058 and 1.79 × 10-10 mm3·N-1·m-1, respectively. In addition, other VB transition metal nitrides (VB TMNs) exhibit the same low friction feature as NbN in the three-phase contact environment; the friction coefficients are even lower than those in an oil-based environment. The water-based lubrication of VB TMNs provides a new reliable scheme for optimizing solid-water composite lubrication systems without additives and is expected to be applied in environments with high humidity or insufficient water coverage.

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