Hybrid nanofilm of exfoliated 2D materials for solid-state lubrication

It has been estimated that globally friction and wear losses is around 20% of the total energy used in industry,1 and this cost includes the consumption of lubricants. A desired solution is the innovation of ultra-low friction surfaces that eliminates the use of fluid lubrication. In a new twist of using layered materials for lubrication, we discovered that nanofilms made of closely tiled exfoliated MoS2 (E-MoS2) flakes exhibit much enhanced low-friction properties and environmental stability when their interparticle space is infiltrated with oil, which are named “oily” nanofilms. We hypothesize a microscopic synergistic effect in such oily films: The oil effectively acts as a binder to hold the exfoliated MoS2 flakes together to form a cohesive continuous lubricious nanofilm, while the closely tiled flakes act as a reservoir to capillarily immobilize the oil, rendering solid thin film appearance and better chemical stability of MoS2 in oxidative and humid environment. The nanofilm works as a continuous 2D oily flake network to protect the wear tracks, offering somewhat self-healing and long-lasting lubrication effect. The oily nanofilms were found to outperform the liquid oil itself in friction reduction and exhibits enhanced environmental stability against humidity, showing the promise as a new type of solid-state thin-film lubricant for various manufacturing processes currently relying on liquid lubricants. The mechanism is likely generic for other 2D materials including graphite, other transitional metal dichalcogenides and h-BN, and the concept of this oily solid-state lubrication opens a new horizon for developing solid-state superlubricity of ultra-low friction.We plan to investigate how such oily MoS2 film works and interacts with substrate under tribological conditions, and extend the discoveries to other 2D materials.