| Polymer materials are widely used in bearings,artificial joint surfaces,space lubrication,engine sealing in the industrial lubrication fields due to their excellent tribological properties and lower cost advantages.However,with the advancement of technology and the continuous expansion of application fields,polymer self-lubricating materials have to face more special and demanding working environment and performance requirements.In order to meet the requirements of low friction coefficient,high wear resistance and strong bearing capacity,polymer materials need to be strengthened and modified.On the other hand,when the polymer material,as a friction pair part,produces friction motion with the counterpart contact surface,some materials in the polymer will be separated from the matrix and transferred to the counterpart friction surface,thus forming a thin transition layer called "transfer film".The transfer film can directly change the frictional contact environment of the polymer material,which indirectly affects the tribological performance of the friction pair parts of the product,and ultimately affects the working performance and service life of the product and equipment.Therefore,it is urgent to study the friction transfer mechanism between surface of polymer friction pairs and the formation mechanism and evolution law of transfer films.In this pape r,experimental analysis and theoretical research were combined to study the friction mechanism of polymer self-lubricating materials and the formation mechanism of transfer film.The main research contents and achievements are as follows:(1)In this paper,hard nanoparticles and soft polyetheretherketone(PEEK)particles were used to modify Polytetrafluoroethylene(PTFE).The enhancement mechanism of soft and hard reinforcing fillers on the tribological performance of PTFE composites and the effect on the properties of the transfer film were investigated.The study found that due to the poor shear resistance of the PTFE matrix,it is easy to produce sliding motion between the molecular chains,and it is difficult to form a complete transfer film on the counterpart friction surface,resulting in extremely poor wear resistance of PTFE.The hard nanoparticles can be embedded into the counterpart friction surface,thereby playing the role of mechanical anchoring of the transfer film,enhancing the adhesion strength of the transfer film,and improving the mechanical and thermodynamic properties of the PTFE composite material.Soft PEEK particles can effectively inhibit the interlayer slip between PTFE molecular chains and can wrap hard nanoparticles,thus reducin g the scratch damage of the transfer film by hard abrasive debris.Therefore,PEEK and nanoparticles can synergistically enhance the tribological properties of PTFE composites.(2)A dynamic ring-on-block friction and wear test device was built,the calculation formula of instantaneous volume wear rate was established,and the ring-on-block friction pair structure was analyzed by finite element method.The research results revealed that the instantaneous volume wear rate could accurately measure the changing trend of material wear during the friction process.Based on this,the friction process of PTFE composites could be divided into: initial wear stage,severe wear stage and stable wear according to the degree of wear stage.According to the finite element analysis,the boundary of the friction interface of the sample was seriously worn.The gradual increase of the friction contact area lead ed to the increase of the retention and rolling time of the debris on the friction interface.These rolling wear debris played the role of "three-body friction".(3)The transfer film samples of PTFE composites in different friction stages were collected.The formation mechanism,influencing factors and evolution rules of the transfer film,as well as the interaction between the transfer film and the tribological properties of PTFE composites were analyzed.The study found that the PTFE composite material filled with nanoparticles and PEEK particles could quickly form a stable and uniform transfer film.The hard nanoparticles were beneficial to improve the film formation rate of the transfer film,and the nanoparticles could chelate and friction sinter with the metal elements on the counterpart friction surface,so as to promote the transfer of the matrix material to the counterpart friction surface and form a thicker transfer film.However,in the later period of friction,nanoparticles had a scraping effect on the transfer film,which destroyed the uniformity and integrity of the transfer film.Soft PEEK particles could effectively inhibit the scratch damage of hard abrasive particles to the transfer film,improve the firmness of the transfer film,and help to maintain the integrity and consistency of the transfer film.(4)The program of image recognition and morphologic al feature quantification was established.Firstly,the transfer film image was enhanced,and the edge the transfer film image was extracted and separated.Then the morphological and texture characteristics of the transfer film were extracted and quantifie d.Finally,a quantitative model of the relationship between the morphological characteristics of the transfer film and tribological properties of the polymer was builted.The study found that the coverage rate of transfer film area,the area of the single transfer film,mean value,third-order moments and consistency were well correlated with the variation trend of wear rate of PTFE composites.Therefore,these five transfer film morphological characteristics could be used as the evaluation basis for the quality of the transfer film,and the friction and wear performance of PTFE composites could be evaluated indirectly.(5)The restricted shear friction model of PTFE composites was established by introducing molecular dynamics.The friction mechanism and transfer mechanism of PTFE composites at atomic scale were studied.The study found that,PTFE exhibited poor shear resistance and obvious interlayer sliding friction in the matrix in the atomic scale friction simulation process,which led to the increase o f internal friction temperature.PEEK could adsorb PTFE and form a firm binding force between the two polymers,thus improving the shear deformation resistance of PEEK/PTFE,inhibiting the sliding friction inside the matrix,and reducing the movement of th e PTFE molecular chain to the iron atomic layer.Nanoparticles could maintain excellent stability and integrity of the PTFE matrix during the friction process,and greatly improve the shear resistance of the PTFE matrix. |
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