Tribological Properties And Mechanism Of Antifriction And Wear Resistance For PTFE Filling Modified By Fluorinated Graphene

Self-lubricating spherical plain bearings play an important role in aerospace and other fields due to their excellent features such as strong bearing capacity,corrosion resistance,and self-lubrication.The effort focusing on the improvement in properties of self-lubricating materials is the key strategy to enhance the quality of self-lubricating spherical plain bearings and to prolong the service life.Polytetrafluoroethylene(PTFE)has an excellent self-lubrication performance,however,the poor mechanical properties and wear resistance of this polymer restrict its application in the field of tribology.Fill modification of PTFE is an effective method to overcome the defects of PTFE,and enhance the tribological properties of this polymer.Unfortunately,although the doping of some fillers(such as silica,alumina and few-layer graphene)can improve the wear resistance of PTFE,the antifriction effect is not obvious or even can deteriorate the self-lubricating properties of PTFE;Therefore,it is still facing challenges to optimize the friction synergistic components to ensure that PTFE possesses excellent antifriction and wear resistance.Fluorinated graphene(FG)possesses high chemical inertness,extreme strength,as well as the easy shear capability,which can help reduce friction and wear,leading to its great application prospects in improving the tribological properties of polymers.Up to now,the most of reported researches have focused on the effects of FG concentration on the tribological properties of polymer composites.However,insufficient efforts have attempted to reveal the influence of fluorine content in FG on the tribological performances of PTFE composites.Therefore,in this paper,the influence of FG and graphene(G)on the tribological properties of PTFE and the mechanism at microscopic friction interface were investigated from the molecular level by molecular dynamics simulation.The results show that FG is more conducive to improving the antifriction and wear resistance of PTFE.In order to deeply study the effects of FG and its fluorine content on the tribological properties of PTFE,four kinds of FG sheets with various fluorine/carbon ratios were prepared,and the tribological properties of FG-filled modified PTFE and PTFE composites reinforced by FG and polyimide(PI)were studied respectively.Furthermore,focusing on the friction interface,the paper not only elucidates the antifriction and wear mechanisms of PTFE self-lubricating composites,but also reveals the evolution of transfer films.Firstly,the friction calculation models consisting of PTFE,G/PTFE,FG/PTFE composites and Fe atomic layers were established,respectively.The friction behaviors between the composites and the metal dual surfaces were simulated and analyzed,then the enhancement mechanisms of the fillers on the interface and friction performance were discussed.Based on the analysis of microscopic datas such as atomic concentration,interfacial binding energy,temperature and velocity in the friction process,it is found that FG can trap more PTFE molecular chains around it,which is benefical to enhance the rigidity of PTFE composites and restrict the movement of PTFE molecular chains.With this condition,only a few of PTFE molecular chains are adhered at the friction interface.In turn,the interaction between PTFE and Fe is reduced,and the energy and temperature at the friction interface are reduced,which is beneficial to reduce the friction coefficients and wear rates of PTFE-based composites.Secondly,using natural flake graphite as raw material,graphene flakes were prepared,and four FG sheets with various fluorine/carbon ratios for enhancing PTFE were prepared by direct fluorination of fluorine-containing gas at different reaction temperatures,and their micro-morphology,molecular structures,and thermal stabilities were studied.It is found that as the degree of fluorination increases,the average particle size and stacked layer of FG gradually decrease,thus increasing its specific surface area.The change in the nature of C-F bonding from semi-ionic to covalent can cause the increase in chemical and thermal stability of FG.Additionally,based on the conclusion of molecular dynamics simulation,the four FG powders and G were filled into the PTFE matrix,respectively,and the influences of fluorine/carbon ratio on the tribological performances of FG/PTFE composites were comparatively investigated.The results indicate that the increase in the fluorine/carbon ratio can help to improve the mechanical properties,thermal stability,and the ability of this composite to resist the shearing force and deformation.Moreover,the semi-ionic C-F bond in the partially fluorinated FG is conducive to the full occurrence of tribochemical reactions,thus contributing to the formation of transfer film,the improvement of the self-lubricity,and the wear resistance of FG/PTFE composite.Finally,in order to further enhance the antifriction and wear resistance of PTFE composites,PI with high strength and high modulus and FG were co-doped into the PTFE matrix to prepare FG/PI/PTFE composites with excellent mechanical and tribological properties.The excellent mechanical strengths and high moduluses of PI and FG can effectively disperse,transfer and support the load and shear force,thereby significantly preventing the slippage,fracture and large-scale damage of the PTFE molecular band structure,promoting the occurrence of tribochemical reactions and the formation of high-quality transfer films.Ultimately,the FG/PI/PTFE composites maintain extremely low wear rates and friction coefficients during the stable wear period.