Study On Mechanical Behavior Of Micro/Nano-scale Contact And Friction Process Using Molecular Dynamics And Multiscale

With the development rapidly of micro/nano technology, MEMS and micro/nano devices has been widely used in aerospace, precision manufacturing, military, biology, agriculture and other fields. During the the scale of micro/nano devices down to the level of micrometer or nanometer, surface force and surface effect resulting form ratio of area to volume play a key role in working performance and failure of micro/nano devices. However, it is not suitable for the research of micro/nanoscale contact and sliding friction by the traditional macro theory of contact and friction. The numerical simulation method of Molecular dynamics (MD) and Multi-scale is an effective method to studying micro/nanoscale contact and friction in today. Therefore, it is great importance that investigating the mechanism of contact and friction in micro/nanoscale for improving performance and lifespan of micro/nano devices. The project is selected from the National Science Foundation of China (NSFC):"Study on methods of multiscale coupling analysis on contact mechanical behavior of Micromechine friction pair". In this study, we simulated contact and friction process for different friction pair such as sphere-spherical, sphere-plane, and investigated the mechanism of contact, friction and abrasion.Firstly, we studied the mechanical behavior of contact process and deformation mechanism of the contact area for monocrystalline silicon between sphere and sphere, sphere and plane using molecular dynamics simulation, then carried out relevant validation on it using Johnson-Kendall-Roberts (JKR) model and atomic force microscope (AFM) test respectively.In order to reveal mechanism of the sliding friction and abrasion in micro/nanoscale, MD simulations are performed to study the sliding friction process between sphere and plane, the surface state changes and friction were discussed for different contact depth in the sliding process, and we analyzed the effect of sliding speed on friction force. In the process of sliding friction, the contact surface of substrate has formed furrow scratches, at the same time the silicon atoms of the hemisphere happened atomic migration and adhered on the substrate surface.According to the AFM tip of different degree abrasion, three-dimensional molecular dynamics simulations are carried out to study the contact and sliding process between the different tip radius of curvature and surface of the single crystal copper. The material deformation and abrasion mechanism are investigated in contact and sliding process. Dislocations defect and emission of the single crystal copper were analyzed for the different curvature radius probe and contact depth in the process by the Centro-symmetry Parameter (CSP) and Common Neighbor Analysis (CNA). The friction was acquired by molecular dynamics simulation and compared with classical friction theory, the ploughing component and adhesion component of friction was investigated. Result shows that a bigger the tip radius of curvature results in a bigger normal force and bigger friction, however, the friction coefficient decreases as the tip radius of curvature.In addition, the friction coefficient increases as the contact depth.For avoiding the MD limitation of spatial and time scale, expanding the model size and improving the computational efficiency. Using Multi-scale method, quasicontinuum (QC), the square head contacted on and pressed in the single crystal aluminum surface, analyzed the contact forces, dislocations nucleation and emission process.Finally, nanoscale contact and sliding experiment of monocrystalline silicon and copper was performed using nanoindentation tester and AFM, results show that the trend of contact force and friction force is consistent with simulation results.