Tribological Behaviors Of PTFE Filled With KEVLAR Fibers Braided Composites

Polytetrafluroethylene(PTFE)filled with KEVLAR fibers braided composites exhibit an attractive combination of excellent strength,low friction coefficient and stable lubricating behaviors.The composites are being used successfully on self lubricating spherical plain bearing which are applied on the key loaded parts in aviation,aerospace and high speed rail fields.It is hard to evaluate the tribological behaviors accurately for homemade materials due to absence of the systematic study on tribological behaviors,so it has being in test stage and has not achieve mass production.In order to evaluate the friction and wear behaviors of homemade self braided composites that there are not enough evaluation methods and facilities under simulated conditions,investigations on tribological behaviors of the self lubricating braided composites are performed using self developed high frequency and pressure oscillating tribological tester.The friction and wear behaviors of the self braided composites are studied as well as the failure mechanisms.And then according to the effects of friction heat on tribological behaviors,friction heat behaviors under different test conditions are studied and the tribological behaviors under different temperatures are analyzed.This could provide experimental and theoretical basis for material application and selection.According to studies presented in the paper main tasks are detailed as follows.Firstly,high frequency and pressure oscillating tribological tester is designed and applied on life-tests to evaluate the friction and wear behaviors of the self lubricating braided composites.Evaluation methods are developed to study the behaviors of different self lubricating braided composites according to service conditions.Under different operating conditions the requirements for tribological behaviors of material are determined and the evaluation criteria are obtained.Secondly,for homemade self lubricating braided composites with excellent behaviors the tribological tester is used to analyze friction and wear behaviors of material under different simulated operating conditions.Microscopic properties of the wear surfaces are presented to compare the friction and wear behaviors,and multiple wear mechanisms occurred during friction process are analyzed in order to select the optimal material.The same type commercial spherical plain bearings are compared experimentally and it is obtained that the friction and wear behaviors of the homemade optimal material are superior to the commercial material.Thirdly,for the optimal braided composites the effects of operating conditions on tribological behaviors are evaluated to investigate the influences of the load and frequency on the friction coefficient,wear loss and friction temperature.It is shown that under high loads(30 MPa~50 MPa)conditions the friction coefficients of the Kevlar fiber braided composites are less affected by the change of the loads and fluctuate within small amplitudes,while the friction coefficients decrease obviously with increasing frequencies.From 5 MPa to 25 MPa the friction coefficients are mainly influenced by loads and little effects of the oscillating frequencies are observed.Fourthly,the effects of the loads and frequencies on wear resistance of the material are analyzed.Full life-cycle wear tests are performed under vaied loads and frequencies to analyze the wear behavior and failure criterion of the braided composites under different conditions.It is indicated that under high loads and high frequencies wear loss of the braided composites are mainly influenced by the oscillating frequencies.The effects of frequencies on the friction coefficient and wear loss are mainly worked by friction temperature.It is demonstrated that the friction temperature has decisive role on friction and wear behaviors.A wear simulation model is used to investigate the contact pressure and wear depth as well as worn state on different contact zones under varing time with changing loads and frequencies.Fifthly,by analyzing the relationship between the friction temperature behaviors of the braided composites and the operating conditions changes of the friction temperature,the friction coefficient and wear loss are obtained.Results show that the friction temperature is not always increased with load,and a decreasing trend is observed during the range of 15 MPa to 20 MPa.Considering the change of the temperature under different operating conditions,wear debris formed during friction process under different temperatures and loads are used to analyze wear mechanisms under different conditions and to explain the mechanisms of the friction and wear affected by friction temperature on a micro level.The investigation could provide basis for practical application and evaluation analysis of the braided composites.Sixthly,the friction and wear investigations on the braided composites under different environment temperatures are performed to obtain the optimal friction temperature.Under this optimal temperature a good lubricating behavior,wear resistance and service life for the spherical plain bearing could be obtained.And this friction temperature could be regarded as a reference value for the dynamic monitoring parameters of this friction pair.The heat conduction parameter are investigated by experiment.The heat conduction behavior of the braided composites is far below from that of its mating material,so the heat are mainly gathered within the braided material and higher friction temperatures are occurred on the contact surface of the friction pair.This phenomenon has obvious influence on the stability and uniformity of the transfer film on the friction surface.Therefore the heat conduction behavior of the braided composites is also an important index evaluating the friction and wear behaviors.Seventhly,a friction heat transfer model for the braided composites is developed to analyze the friction heat and wear mechanism.During friction process the heat transfer and the temperature distribution on the friction surface are analyzed by micro heat transfer model.Because of the stress and strain of the friction pair the highest temperature of the friction surface is not occurred at the central position,but is occurred at either end.So both ends of the braided composites are easily worn failure.