Space environment is greatly different from atmospheric environment in many aspects, e.g. energetic atomic oxygen, ultra-high vacuum, microgravity, alternate extreme temperature, solar UV radiation, and this produces a vital influence on aerospace devices’ reliability and life durability. In space environment, the vacuum and microgravity environments always induce high friction forces between contact surfaces. In vacuum, adhesive effects induce a high adhesion component of a friction force. Under microgravity, the irregular collision between contact bodies induced by the disturbance occurs all the time, which has a great influence on a ploughing component. The kinetic energy of a contact body will be converted to the mechanical energy during the collision

process, and the friction forces could be underestimated if the collision effects are neglected. As a result, the adhesive and collision effects should be considered simultaneously during the simulation of friction behaviors under space environment. As one of the most widely used solid lubricants in space environment, molybdenum disulfide (MoS₂) can significantly improve the tribological properties of friction pairs. Moreover, soft metals are always used as solid lubricants in spacecraft, and a soft metal Ag could be used to weaken the collision effects. The combination of MoS₂ and Ag may provide synergistic lubricating performance. To investigate the influence of MoS₂/Ag films on the friction behaviors, a monolayer MoS₂ film was coated on the Ag surface to form MoS₂/Ag films. It is difficult to perform microgravity friction experiments on the earth, and the chance to take a friction experiment in space is scarce. Therefore, in this work, based on the motion characteristics of a space mechanism, a coarse-grained molecular dynamics method was used to study the friction properties of collision sliding contacts between a rigid indenter and the MoS₂/Ag films considering the influence of the vibration in space environment. A coarse-grained molecular dynamics model of the collision sliding contacts in the vibration environment was developed. The friction properties of bare Ag and MoS₂/Ag films were compared, and the effects of collision velocity, sliding velocity and space temperature on the friction process of the collision sliding contacts were investigated. The results showed that the MoS₂/Ag films showed better friction properties compared to the bare Ag. MoS₂ films had a high specific surface area and out-of-plane strength so that they can enhance load-bearing capacity and reduce deformation of the substrate. The collision velocity of the indenter made a certain contribution to the kinetic energy, and the increase of the collision velocity increased the indentation depth, which led to the increase of the average friction force. By increasing the sliding velocity, the shear between substrate atoms was enhanced, and the average friction force increased. At high sliding velocity, the rearrangement of dislocation atoms in the shear area was impossible, resulting in that a large number of atoms were accumulated in the shear area, and consequently the atom density in the shear area was increased and the mutual squeeze and shear friction between the atoms were intensified. Besides, alternate extreme temperature was a major feature of the space environment. Due to high vacuum in the space environment, there is little atmospheric convection and heat conduction, and the spacecraft has to enter or exit the shadow of the earth frequently, which makes friction pairs exposed to extreme temperature. MoS₂/Ag films showed excellent friction properties in the temperature range of 100~500 K, while their friction forces were increased when the ambient temperature was 600 K, and accompanied by the rupture of the MoS₂ film. The research of this paper provided a theoretical basis for the friction reduction in the space environment.