ABSTRACT
Owing to nano-MoS2's excellent anti-friction and anti-wear properties, nano-MoS2, which can act as a nano-additive in lubricating oil or solid lubricants, is believed to have great potential in the lubrication of power machinery and moving parts of a spacecraft. The molecular dynamics method was used to construct a rough surface and a multi-level asperity structure to simulate starved lubrication before oil film breakdown, and the lubrication mechanism of MoS2 as a nano-additive or directly coated on the textured surface could reduce the friction coefficient and wear was explained from the atomic perspective. Simulations showed that the multilayer MoS2 played a role of load-bearing at light load or low velocity, and slipped into the grooves to repair the surface under heavy load or high velocity. Even if local asperity contact occurs, MoS2 nanoparticles could accelerate the detachment of the initial asperity contact to prevent large-scale adhesion. The MoS2 nanoparticles transformed the pure liquid oil film into a liquid-solid composite oil film, which was more suitable for lubrication under heavy load and high velocity because it increased the contact area, protected the friction surface and prevented asperity contact. The proposed lubrication mechanism contributes to understanding the frictional properties of layered nanomaterials under extreme conditions and provides a reference for further application of MoS2 materials in the field of lubrication.
ABSTRACT
Non-equilibrium molecular dynamics simulations of liquid flow through the surface were performed to investigate the flow resistance and thermal resistance under conditions of different solid-liquid interactions and surface temperatures. A novel phenomenon was observed in the simulation, namely the rise of surface temperature increases the flow resistance when solid-liquid interaction is weak, but decreases the flow resistance when solid-liquid interaction is strong. A higher density of the boundary layer brings a larger friction force to increase the flow resistance. For heat transfer, it is innovative to calculate the heat conduction and convection of the boundary region discretely. The results showed that the heat transfer performance of the interface is not positively correlated with the boundary liquid density, and the structure of the boundary liquid is also crucial. We believe that this research can improve the existing theory of flow heat transfer and provide a more effective method for analyzing the flow heat transfer of the solid-liquid interface.
ABSTRACT
Experiments and simulations have shown that molybdenum disulfide (MoS2) has unique mechanical and electrical properties that make it promising for application as a flexible material in microscopic and nanoscopic electronic devices. In this paper, the molecular dynamics method is used to study the mechanical properties of multilayer MoS2 during compression and stretching under different intra-layer and inter-layer potentials to choose the most suitable ones. The results show that the increase in the inter-layer repulsive force during compression was all provided by sulfur atoms in the adjacent layer. The two intra-layer potentials represented two forms of tensile fracture: plastic fracture with structural holes or lattice distortions, and brittle fracture with instantaneous destruction. The chosen inter-layer potential had a significant influence on the structure of the multilayer MoS2 but the effect of inter-layer potential during stretching was not prominent. By comparing these results with reference values, the most suitable intra-layer and inter-layer potentials for the multilayer MoS2 were selected, and can serve as a reliable reference for subsequent simulations.
ABSTRACT
To reveal the friction mechanism of molybdenum disulfide (MoS2), the frictional properties of multilayer MoS2 lubrication film were studied under variable loads and shearing velocities by the molecular dynamics (MD) method. The results showed irreversible deformation of MoS2 was caused by heavy load or high shear velocity during the friction process and the interlayer velocity changed from a linear to a ladder-like distribution; thus, the number of shear surfaces and the friction coefficient decreased. The low friction coefficient caused by heavy load or high velocity could be maintained with a decrease in load or velocity. For a solid MoS2 lubrication film, the number of shearing surfaces should be reduced as much as possible and the friction pair should be run under heavy load or high shear velocity for a period of time in advance; thus, it could exhibit excellent frictional properties under other conditions. The proposed friction mechanism provided theoretical guidance for experiments to further improve the frictional properties of MoS2.
