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In this paper, a collision friction model for a double-layer MoS2 film is proposed considering the microgravity induced collision in space environment. A modified REBO (Reactive Empirical Bond Order) potential is used to describe interactions among the atoms in the MoS2 film. The collision friction process of the MoS2 film is simulated by vibration...
Citations
... Dang et al. [24] investigated the failure mechanisms of an indented single-layer MoS2 by using molecular dynamics simulations. Tong et al. [25] performed molecular dynamics simulations of the collisional friction properties of MoS2 in microgravity, and they investigated the effects of collision frequency and amplitude on the frictional properties of MoS2 thin films. Hu et al. [26] investigated the friction properties of multilayer MoS2 by molecular dynamics, and they found that heavy load and high shear rate would lead to irreversible deformation of MoS2, changing the interlayer velocity from a linear distribution to a stepped distribution. ...
Aiming at the problem of the failure of bearing lubrication caused by the entry of fine dust into the spacecraft, a molecular dynamic simulation method was proposed to study the wear characteristics of the bearing lubricating film. Firstly, the atomic model of MoS2 thin film is established, and the motion state of dust particles on the surface of MoS2 thin film is divided (sliding abrasive and rotating abrasive). Secondly, the characteristics of sliding and rotating abrasive wear are studied by applying different loads to the abrasives. Finally, the surface structure of the substrate was changed to explore the adsorption and wear characteristics of the one-dimensional rough substrate. The following conclusions are obtained: (1) sliding abrasive and rotating abrasive have different wear mechanisms. The wear of sliding abrasive originates from the accumulation and release of tangential force. The main source of wear in rotating abrasive is the release of tangential force and the action of tangential force caused by rolling; (2) The relationship between sliding friction and rolling friction at the atomic scale is the same as that at the macroscopic scale. Rolling friction is much less friction than sliding friction. Rolling friction causes less damage to the film than sliding friction; (3) The one-dimensional rough substrate reduces the contact area between the film and the film, resulting in a decrease in the adsorption between the film and the substrate. The asperity structure of rough substrates reduces the lubricity of the film compared to smooth substrates.
... Another approach is to treat irregular disturbances as vibrations. The effects of vibrations on the friction force have been studied by many researchers Leus 2012, 2015;Jadav et al. 2018;Tong et al. 2021). Yoo and Kim (2016) accessed the effects of vibration frequency, amplitude, and direction on friction reduction through experiments. ...
Friction is a primary failure mode in micro-nano electromechanical systems due to the high surface-to-volume ratio. Microgravity further complicates this issue in journal-bearing-like conformal contacts by promoting irregular disturbances. This paper aims to gain insights into the anti-friction design of journal-bearing-like devices through molecular dynamics simulation. A molecular dynamics model was proposed and the calculation method of the friction force was derived. In the absence of disturbance, the proposed model was compared with a non-conformal model which unfolded the bearing as a plane, and the influence of initial radial clearance and axis inclination on the friction force was investigated. The results showed that the proposed model could present more accurate friction forces than the non-conformal model. The friction force was inversely proportional to the initial clearance, and the axis inclination could reduce the friction force. Regarding disturbances as the superposition of two vibrations perpendicular to each other, in which case the trajectory of the journal was a Lissajous curve, the effects of frequency, stiffness coefficient, amplitude ratio, and frequency ratio were investigated. The results showed that the average friction force increased with the rising frequency in the range of 0.8 ~ 4.8 GHz, then decreased with the further increase of frequency. The average friction force was lowered when the stiffness coefficient increased from 100N/m to 1000N/m. For two representative frequencies, the average friction force exhibited different trends with the amplitude ratio. Except for the case of 1.25, increasing the frequency ratio could reduce the friction force. It seemed that applying a well-designed Lissajous route was a promising way to reduce friction.
... Tong et al [11]. performed molecular dynamics simulations of the collisional friction properties of MoS2 in microgravity, and they investigated the effects of collision frequency and amplitude on the frictional properties of MoS2 thin films. ...
Aiming at the problem of the failure of bearing lubrication caused by the entry of fine dust into the spacecraft, a molecular dynamic simulation method was proposed to study the wear characteristics of the bearing lubricating film. Firstly, the atomic model of MoS2 thin film is established, and the motion state of dust particles on the surface of MoS2 thin film is divided (sliding abrasive and rotating abrasive). Secondly, the characteristics of sliding and rotating abrasive wear are studied by applying different loads to the abrasives. Finally, the surface structure of the substrate was changed to explore the adsorption and wear characteristics of the one-dimensional rough substrate. The following conclusions are obtained: 1) sliding abrasive and rotating abrasive have different wear mechanisms. The wear of sliding abrasive originates from the accumulation and release of tangential force. The main source of wear in rotating abrasive is the release of tangential force and the action of tangential force caused by rolling; 2) The relationship between sliding friction and rolling friction at the atomic scale is the same as that at the macroscopic scale. Rolling friction is much less friction than sliding friction. Rolling friction causes less damage to the film than sliding friction; 3) The one-dimensional rough substrate reduces the contact area between the film and the film, resulting in a decrease in the adsorption between the film and the substrate. The asperity structure of rough substrates reduces the lubricity of the film compared to smooth substrates.
... Tong et al [11]. performed molecular dynamics simulations of the collisional friction properties of MoS2 in microgravity, and they investigated the effects of collision frequency and amplitude on the frictional properties of MoS2 thin films. ...
Aiming at the problem of the failure of bearing lubrication caused by the entry of fine dust into the spacecraft, a molecular dynamic simulation method was proposed to study the wear characteristics of the bearing lubricating film. Firstly, the atomic model of MoS2 thin film is established, and the contact state of dust particles on the surface of MoS2 thin film is divided (two-body contact state and three-body contact state). Secondly, the characteristics of two-body and three-body abrasive wear are studied by applying different loads to the abrasives. Finally, the surface structure of the substrate was changed to explore the adsorption and wear characteristics of the one-dimensional rough substrate. The following conclusions are obtained: 1) Two-body contact and three-body contact have different wear mechanisms. The wear of two-body contact originates from the accumulation and release of tangential force. The main source of wear in three-body contact is the release of tangential force and the action of tangential force caused by rolling; 2) The relationship between sliding friction and rolling friction at the atomic scale is the same as that at the macroscopic scale. Rolling friction is much less friction than sliding friction. Rolling friction causes less damage to the film than sliding friction; 3) The one-dimensional rough substrate reduces the contact area between the film and the film, resulting in a decrease in the adsorption between the film and the substrate. The asperity structure of rough substrates reduces the lubricity of the film compared to smooth substrates.
... Tong et al [11]. performed molecular dynamics simulations of the collisional friction properties of MoS2 in microgravity, and they investigated the effects of collision frequency and amplitude on the frictional properties of MoS2 thin films. ...
Aiming at the problem of the failure of bearing lubrication caused by the entry of fine dust into the spacecraft, a molecular dynamic simulation method was proposed to study the wear characteristics of the bearing lubricating film. Firstly, the atomic model of MoS2 thin film is established, and the motion state of dust particles on the surface of MoS2 thin film is divided (sliding abrasive and rotating abrasive). Secondly, the characteristics of sliding and rotating abrasive wear are studied by applying different loads to the abrasives. Finally, the surface structure of the substrate was changed to explore the adsorption and wear characteristics of the one-dimensional rough substrate. The following conclusions are obtained: 1) sliding abrasive and rotating abrasive have different wear mechanisms. The wear of sliding abrasive originates from the accumulation and release of tangential force. The main source of wear in rotating abrasive is the release of tangential force and the action of tangential force caused by rolling; 2) The relationship between sliding friction and rolling friction at the atomic scale is the same as that at the macroscopic scale. Rolling friction is much less friction than sliding friction. Rolling friction causes less damage to the film than sliding friction; 3) The one-dimensional rough substrate reduces the contact area between the film and the film, resulting in a decrease in the adsorption between the film and the substrate. The asperity structure of rough substrates reduces the lubricity of the film compared to smooth substrates.
A considerable portion of space mechanism failures are related to space tribological problems. Cold welding in high vacuum; surface erosion and collision damage caused by various radiations, high temperature oxidation under atomic oxygen (AO) bombardment; and thermal stress caused by temperature alternation all alter the physical, chemical, and friction properties of materials. In particular, the space vibration caused by alternating temperatures and microgravity environments can alter the motion of the contact body, further affecting its friction properties. Improving the friction properties of contact surfaces in the space environment is an important way to extend the service life of spacecraft. Traditional lubricants can no longer meet the lubrication requirements of the space environment. This study describes the characteristics of the space environment and the applications of solid lubricants. The friction properties of MoS 2 , a solid lubricant widely used in space, are discussed. The synergistic lubrication of MoS 2 with surface textures or metals is presented. Advances in research on the friction properties of collision sliding contacts in the space environment are reviewed. The combination of MoS 2 and soft metals with surface textures is introduced to reduce the effects of vibration environments on the friction properties of moving parts in space mechanisms. Finally, the challenges and future research interests of MoS 2 films in space tribology are presented.
Soft metals are often used for space mechanism lubrication because of their low shear strength. In outer space, the vibration of spatial mechanism will occur when there is a small disturbance due to the effects of microgravity environment. Studies on the friction properties of soft metals in vibration environment could contribute to the application of space lubrication materials. Taking a clearance joint as an example, the relative motion between the shaft and the bearing is simplified to a sliding contact between a cylinder and two smooth contact bodies. A molecular dynamics model of the collision sliding contact between a rigid cylindrical indenter and an elastic substrate is established. The effects of sliding velocity, collision velocity and indenter radius on the friction properties of soft metals are studied. The results show that the Ag substrate and Au substrate present strong adhesion to the Fe indenter. The indenter and the substrate are always in a state of adhesive sliding contact. The larger the initial collision velocity of the indenter, the higher the friction force. The friction force shows great values as the sliding velocity increases. As the increase of indenter radius, the contact area is enlarged, which results in a high friction force. The adhesion of the Cu substrate to the Fe indenter is weak, so the friction force shows a low value, and the friction performance of Cu is the best, while the friction performance of Au is the worst.
With the development of nanotechnology, micro-nano electromechanical systems are widely used in various high-precision instruments, e.g., micro-nano satellites, pico-satellites, in which single-crystal Si is always employed. In outer space, there will be random collisions in space mechanisms caused by unexpected and irregular small disturbances especially under the microgravity environment. In this paper, a molecular dynamics model is used to simulate the motion of the mechanism based on the motion characteristics of random collisions happened within space mechanisms, and irregular collisions are represented by sinusoidal vibrations of the indenter in Y and Z directions. The friction properties of the collision sliding contacts under the lubrication of gold films with different layers are investigated. The results show that a nine-layer gold film presents good vibration absorption and lubrication performance. Then, rectangular textures are designed and made on the gold film. The textured surface can significantly reduce the friction forces at high vibration frequencies. In addition, the friction properties of a molybdenum disulfide (MoS2) film and the combined lubrication behaviors of molybdenum disulfide films and gold films are studied. The molybdenum disulfide film shows excellent friction properties at different vibration frequencies, and the combination of molybdenum disulfide and gold films can significantly reduce the average friction force within a specific vibration frequency range.
Interatomic potentials are used to describe the motion of the individual atoms in atomistic simulations. A perfect approach of the interatomic forces in a system of atoms requires massive quantum mechanical calculations, which are not computationally viable in large-scale simulations. This chapter discusses the developmental history of Tersoff and REBO potentials along with the challenges and limitations associated with these potentials. Also, a glimpse of their advantages, usage, and applications have been presented.KeywordsInteratomic potentialsMolecular dynamicsMechanical propertiesAtomistic scale simulations
In this work, the metal phase MoS2 was prepared by a simple one-step hydrothermal method, and it was systematically studied by XRD, Raman, SEM, TEM, and other characterization methods. In addition, the tribological behavior of M-MoS2 in water-based drilling fluids has been extensively studied with a ball-disk tribometer. In addition, the influence of applied load and speed on friction performance is also studied. The experimental results show that the introduction of M-MoS2 significantly reduces the friction and wear of the material. Among them, M-MoS2-5-water-based drilling fluid has the lowest friction coefficient (~0.11).
