Molecular Dynamics Simulation on the Vacuum Tribological Properties of Polytetrafluoroethylene

With the development of aerospace industry, the space tribology of polymer materials has attracted extensive research interest and great attention. Polytetrafluoroethylene (PTFE) as a common solid lubricating material and lubricant is widely used in space bearings and seals. The common research methods for study the effects of space environment on the tribological properties of typical polymer composites were space exposure experiments and ground simulation experiments. However, the cost of space exposure experiment and ground simulation experiment is too high. In this study, we employed a molecular dynamics (MD) simulation to study the tribological properties of PTFE and iron sliding models under different vacuum degrees. Firstly, the molecules model of PTFE with 10 chains containing 50 repeat units of C₂F₄ was built using Material Studio software, and the Condensed-Phase Optimized Pote\ntials for Atomistic Simulation Studies (COMPASS) force field was used. After the geometry optimization, a series of MD simulations were sequentially performed, which contained three processes of a 10-circle thermal annealing, NVT ensemble and NPT ensemble. Then, the frictional model of PTFE sliding against iron was built and the tribological properties was evaluated. The simulated results showed the average friction coefficient of PTFE under 1.0×10⁻⁴ GPa was 0.073, and the average friction coefficient of PTFE under vacuum degree of 1.0×10⁻⁹ GPa was 0.061, which was 16.4% lower than that under atmospheric pressure. The average friction coefficient of PTFE was 0.042 when the vacuum degree was 1.0×10⁻¹² GPa, which was 42.5% lower than that at normal pressure. This indicated that the friction coefficient of PTFE decreased with the increase of vacuum. Through the molecular dynamics friction snapshots of PTFE at different times during the friction process, it was found that PTFE was prone to produce wear debris during the friction and shear process at atmospheric pressure, resulting in wear debris. In the high vacuum state, the adhesion of PTFE molecular chain to the dual surface was easy to occur after the molecular chain breaks. By counting the number of broken atoms in the graph, the wear rate of PTFE was 45.2% under atmospheric pressure and 54.3% under vacuum of 1.0×10⁻⁹ GPa. When the vacuum degree was 1.0×10⁻¹² GPa, the wear rate of PTFE was 57.6%. The results showed that the wear of PTFE increased obviously with the increase of vacuum degree. By analyzing the changes in the radial distribution function (RDF), relative concentration of atoms, friction interface temperature and atomic movement speed, it was found that with increasing of the vacuum degree, more PTFE molecular chains were adsorbed on the surface of iron atom layer, which enhanced the interaction, increased the friction interface temperature and accelerated the atomic movement speed. In the sliding process, more PTFE molecules adhered to the surface of iron atom, resulting in increased wear. Moreover, the simulation results were further verified by friction tests, which were evaluated using a ball-on-disc tribometer at room temperature in a vacuum level of 1.0×10⁻⁴, 1.0×10⁻⁹ and 1.0×10⁻¹² GPa. The friction test results of PTFE under different vacuum degrees showed that with the increase of vacuum degree, the friction coefficient of PTFE gradually decreased and the wear rate increased. Which verified the reliability of molecular dynamics simulation calculation results.