Self-lubricating journal bearings are widely used in the aerospace field due to their compact structure and strong load bearing capacity.The tribological properties of self-lubricating materials are crucial to the service life of self-lubricating journal bearings.Polytetrafluoroethylene(PTFE)is a self-lubricating material with low surface energy,which facilitates relative sliding between molcules and allows them to adhere to the contacting surfaces to form transfer films for achieving friction reduction.However,the mechanism of transfer film formation during friction remains to be further explored.Furthermore,PTFE undergoes friction induced chemical reactions during complex working conditions,resulting in structural changes and the formation of cross-linked PTFE.This paper takes PTFE and PTFE/graphene composites as examples to study the mechanism of transfer film formation during the friction process and the influence of molcular structural changes on the tribological properties of the materials.In this paper,friction models of PTFE and PTFE/graphene composites were constructed based on Materials Studio software.The XRA12 self-lubricating journal bearing was taken as the research object to establish the relationship between the load and the contact stress.The contact stress was applied to the friction model.By analyzing the conformational changes of PTFE molcular chains,center of mass displacement,contact area,and other parameters during the friction process,the formation mechanism of transfer films was revealed.With the increase of load,the thickness of transfer film formed by PTFE and PTFE/graphene composites increases,but it will aggravate the rupture of transfer film.PTFE/graphene composite is more stable than the transfer film formed by pure PTFE.Based on the molecular dynamics theory,PTFE/graphene composite models with different crosslinking degrees were established to simulate the friction process between PTFE/graphene composite and upper Fe under different crosslinking degrees.The influence of crosslinking degree on tribological properties of PTFE/graphene composites was analyzed by analyzing the change of matrix energy and wear depth during friction.The effects of different loads on the tribological properties of crosslinked PTFE/graphene composites were also studied based on molecular dynamics.With the increase of loads,the friction coefficient gradually decreased and the wear depth gradually increased.By analyzing the non-bond interaction and bond interaction between atoms at the friction interface,the effect of load on the friction coefficient is revealed,and it is found that the bond interaction has more influence on the friction force than the non-bond interaction.The effect of load on wear was analyzed by studying the depth of wear and MSD variation of molecular chain along z direction.To validate the simulation results,PTFE/graphene composites with a graphene content of 2wt.% were prepared using techniques such as solution blending and cold pressing-sintering.Using the UMT-3 friction and wear tester,the effects of load and speed on the friction coefficient and wear of the PTFE/graphene composite were analyzed based on the wear morphology and SEM images. |