Macroscale superlubricity enabled by rationally designed MoS2-based superlattice films

Although superlubricity is highly desirable for many engineering applications, its implementation has so far been seriously restricted due to limitations in contact size, environmental adaptability and life time. By designing superlattice films with alternating molybdenum disulfide (MoS2) and tungsten carbide (WC) layers, we show that long-term macroscale superlubricity (friction coefficient of 0.006) in low vacuum (~10-3 Pa) after a short running-in period in air. Such unusual behavior is enabled when the fine structure of the bilayer unit is rationally controlled to yield incommensurate contacts between MoS2 and metal oxides nanoparticles produced spontaneously during tribological sliding. Our analysis indicates that the WC phase is critical for superlubricity by helping stiffen the film, facilitate preferential growth of crystalline MoS2 along (002) plane parallel to substrate, and produce lubricous nanoparticles. We further demonstrate the superlattice design is generally applicable for MoS2/ceramics materials to achieve long life-time macroscale superlubricity with easy self-rejuvenation capability.