Currently, energy losses due to friction and wear account for about one-third of all human energy consumption. Achieving extremely low friction not only reduces energy consumption but also extends mechanical life. Superlubliricty, defined as a state in which friction is close to zero (friction coefficient less than 10-3) when two solid surfaces are in contact, has been a cutting-edge research direction in tribology since its discovery in the 1990s. Structural superlubliricty is an important means to achieve superslipticity, in simple terms, if you do not consider the chemical bonds between two crystals touching atoms, structural supersliptting should be looked for from the non-metric interface.
It is worth noting that the layered materials that exist in nature, such as graphite, molybdenum disulfide, hexagonal boron nitride, etc., have been used as natural solid lubricants for more than 100 years due to the weak van der Waals force between layers. In principle, the non-metric van der Waals interface is an ideal system for studying structural ultra-slip. How to construct such an interface has become a remarkable problem in the field of ultra-slip research in recent years. In 2004, Joost W.M. Frenken's team at Leiden University in the Netherlands took the lead in measuring extremely low friction between two layers of graphite with a certain angle (Phys Rev Lett 2004, 92, 126101), showing a classic example of structural super-slip. In 2008, a team of academician Zheng Quanshui of Tsinghua University discovered the self-shrinkage effect caused by extremely low friction in a similar interface (Phys Rev Lett 2008, 100, 067205). In this homogeneous van der Waals interface, the zero angle contact is the metric, resulting in the maximum friction; the small corner contact will produce a Mohr pattern, and the local lattice in the Mohr cycle also has a publicity situation, resulting in a large friction; the large angle contact is approximately non-metric, resulting in extremely small friction.
To get rid of the corner dependence on friction in the van der Waals interface, one possible solution is to construct the van der Waal heterogeneous interface. In 2016, the Zhang Guangyu team from the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics studied the thermal stability of the graphene-hexagonal boron nitride interface and found that the thermogenesis rotation phenomenon (Phys Rev Lett 2016, 116, 126101) revealed that this van Der Waals heterogeneous interface may have structural superslikness. In 2018, a team of academician Zheng Quanshui of Tsinghua University measured its interfacial friction and verified the structural super-slip phenomenon, and found that the friction anisotropy observed in the heterojunction was much lower than the data measured in its homogeneous part (Nature Materials 2018, 17, 894). However, due to the small lattice mismatch between graphene and hexagonal boron nitride (~1.7%), this van der Waals heterojunction still has a spike effect due to the Mohr superlattice at small corners. Revealing stable, isotropic structures at different corners remains a challenge.
Recently, the Zhang Guangyu team from the Institute of Physics of the Chinese Academy of Sciences/Beijing National Research Center for Condensed Matter Physics collaborated with the Tomas Polcar team of the Czech University of Technology to systematically study the supersliptic phenomenon of the heterogeneous interface of the large lattice mismatch van der Waals. Liao Mengzhou, a doctoral student supervised by Zhang Guangyu, used environmentally controlled atomic force microscopy technology to study the intrinsic friction characteristics of the epitaxial growth of molybdenum disulfide-graphite (lattice mismatch ~26.8%) and molybdenum disulfide-hexagonal boron nitride (lattice mismatch ~24.6%), and found that the two large lattice mismatch van Der Waal heterogeneous interfaces are ideal supersliptic systems, not only the friction coefficient can be lower than 10-6 (and the measurement limit is at the same level), and do not rely on the mutual angle between the layers (that is, isotropy).
Figure 1: Measurement method of friction force at van der Waals interface.
Figure 2: The phenomenon of super-slip in the interface of the great mismatch van der Waals.
Further studies of sample sizes revealed the source of friction and found that there was close to zero friction in the face of this large lattice mismatch van der Waals heterojunct, and the nailing effect of the boundary was the main source of overall friction. As a comparative sample, in the small lattice mismatch van der Waals heterojunct (graphene-hexagonal boron nitride), intra-plane interface friction still dominates.
Figure 3: Exploration of the source of friction.
To explain the mechanism of the boundary nailing effect, Dr. Paolo Nicolini of Tomas Polcar's team conducted molecular dynamics simulations of this. The simulation results show that the bounding of boundary atoms is weakened due to the periodicity of the lattice being broken at the boundary. Therefore, the boundary atom is more active than the central atom. At the same time, the calculation results also show that the potential energy surface of the boundary atom is significantly higher than that of the central atom. Molecular dynamics simulations clearly show the different states of boundary and central atoms, and more unstable boundary atoms are more likely to dissipate energy in slip.
Figure 4: Molecular dynamics simulation of the MoS2/graphite interface.
This work experimentally explores the isotropic superslik phenomenon in the heterogeneous interface of a large lattice mismatch van der Waals, which provides a new idea for the design and application of superslik interface. At the same time, the work shows that structural defects such as lattice boundaries, interface steps and dislocations should be avoided as much as possible in the interface supersliptic system, so as to reduce its negative effect on structural superslipticity.
Figure 5: The effect of the interface step on the friction.
The relevant research results were published in Nature Materials 2021, and the work was supported by the key projects of the National Natural Science Foundation of China, the Class B Pilot Project of the Chinese Academy of Sciences, and the Songshan Lake Materials Laboratory.
Edited: It's not easy to see the mountains, yr Lewis