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In nanomaterials, the surface or the subsurface structures influence the friction behaviors greatly. In this work, nanoscale friction behaviors between a rigid cylinder tip and a single crystal copper substrate are studied by molecular dynamics simulation. Nanoscale textured surfaces are modeled on the surface of the substrate to represent the surface structures, and the spacings between textures are seen as defects on the surface. Nano-defects are prepared at the subsurface of the substrate. The effects of depth, orientation, width and shape of textured surfaces on the average friction forces are investigated, and the influence of subsurface defects in the substrate is also studied. Compared with the smooth surface, textured surfaces can improve friction behaviors effectively. The textured surfaces with a greater depth or smaller width lead to lower friction forces. The surface with 45° texture orientation produces the lowest average friction force among all the orientations. The influence of the shape is slight, and the v-shape shows a lower average friction force. Besides, the subsurface defects in the substrate make the sliding process unstable and the influence of subsurface defects on friction forces is sensitive to their positions.

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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.

Background: In space environment, microgravity and vacuum influence the mechanical behaviors of the devices. In microgravity environment, the mechanical components will vibrate with a small amplitude once there is a disturbance. The vibration can be seen as a reciprocating sliding contact with a small amplitude. In addition to the vibration, adhesion effects are predominant in vacuum, which will induce a high friction force. Objective: To reduce the friction force, textured surfaces are widely used in mechanical engineering on the earth, and nanoscale textures are also verified that they can be used to improve the frictional behaviors of components with the size of nanometers. Methods: In this paper, the adhesion effects are considered by using molecular dynamics (MD) simulation, and the microgravity induced vibration is simplified as a reciprocating sliding contact. Coupling MD simulation and the finite element method, a multiscale method is used to investigate the frictional properties of nanoscale reciprocating sliding contact between rigid multi-asperity tips and an isosceles trapezoid textured surface. Results: Average friction forces for the different tips are presented, and the friction processes are analyzed. A stable friction process is discovered for a specific case, and the average friction forces keep at two stable values corresponding to two sliding directions. Conclusion: Compared with the total average friction forces of a smooth surface, the textured surface can reduce the friction forces greatly. This work could contribute to the textured surface design to improve frictional properties in space environment.