Recent development in surface/interface friction of two-dimensional black phosphorus: A review

In 2014, with the development of synthesis and modification methods of black phosphorus (BP), single or multiple layers of BP were stripped into two-dimensional (2D) layered materials, which had great prospects in transistors, batteries, optoelectronics, friction, and lubrication fields. From this point of view, we highlight recent advances in BP research, particularly its tribology and lubrication properties. This paper introduces mainly the research progress of BP in the solid-liquid lubrication fields, and systematically expounds its friction nature from the perspective of macroscopic, microscopic, and computational tribology. Under special conditions (high load, oxidation, etc.), a long-term superlubricity performance of BP could be obtained, which far exceeded other traditional 2D lubrication materials (Gr, MoS2, etc.). There were obvious deficiencies and misunderstandings about the macroscopic and microscopic superlubricity mechanism of BP lubricant, due to the complex and diversified frictional interfaces. The superlubricity mechanism of BP was roughly attributed to the multi-factor coupling or synergistic action in macroscopic, and it was still an open question whether there was secondary transition or contact area difference of the friction interface in microscopic. We believe that these deficiencies and misunderstandings are more ascribed to the lack of research on the interface transition behavior and mechanism during BP friction. We analyze and summarize the challenges and limitations in understanding BP's superlubricity mechanism based on macroscopic and microscopic experiments in the current BP friction research. Finally, we propose a computational tribology-based approach to reconcile discrepancies between macro- and micro-scale experiments.