| J.Krim et al.had studied the interface friction of Pb surface and nitrogen molecule adsorbed on it with quartz crystal microbalance(QCM) from 1986.They observed the friction dropped abruptly when the temperature was cooled to the superconducting transition temperature(Tc=7.2K) of Pb,and reported the electronic contribution to friction.Its significance is to link the phenomenon of the macrofriction and the micro-atomic structure.Then Kuleba et al.also studied effect of superconducting transition on tribological properties of metal superconductor and superconducting ceramics,and validated the theory by the test in low temperature with thin films of many metal such as Ag and Au.From theory,the test of Krim et al. was special design.It was calculated friction force associated with slippage of molecularly thin nitrogen films adsorbed on a metal substrate with differential equation of oscillation and formula of quantum mechanics according to the frequency shift with temperature.The energy dissipative was not considered(called interfacial friction).The purpose of the research is that whether the contribution of the electron to the friction is observed on common friction,and how to make use of the phenomenon to design the new solid lubricating composite.From the application point of view,Many component of space machinery was working in vacuum,the high temperature,the low temperature and particular corrosion medium.For example,the pump bearing of liquid hydrogen and liquid oxygen was working under the absolute humidity of 100K.Therefore,the problem about lubrication under exceeding running conditions is an important subject in tribology field.And with the rapid development of science and technology,the request to the function of friction material was more and more high.The traditional friction material has not satisfied the actual request.From practicality,the superconducting transition temperature of pure metal is Several K(in liquid helium),and Application of high cost.However,the superconducting transition temperature of high temperature superconducting material (Bi,Pb)2Sr2Ca2Cu3Ox(Bi2223) is above 100K,which can be realized at liquid nitrogen temperature.It reduced costs and has practical significance,so it selected as our research material.In this study,the friction characteristics of Bi2223 was evaluated in the self-made sales - disk ultra-low temperature friction testing machine and found that the friction coefficient was lower and stability in the point of superconducting transition.However,the friction coefficient of Bi2223 is higher and instability at room temperature.In order to obtain a wide range temperature of low and stable friction coefficient material,the Bi2223-Ag composites was designed.This study was using powder metallurgy method to prepare Bi2223-Ag self-lubricating composite with uniform and compact structure.The microstructure was studied by XRD and SEM. Metal Ag powder was doping in.The mechanical properties were tested and the tribological properties were investigated in low temperature,air temperature,vacuum and high temperature.The toughness and tribological mechanism were discussed.Various amounts of Ag additions in the microstructural evolution and related mechanical and tribological properties of Bi2223 were studied.The results showed:(1) The density and fracture toughness of composites increased with the increase of Ag content,and the micro-hardness decreased with the increase of Ag content, probably as a result of strengthening mechanism caused by Ag addition.Ag particles may resist crack propagation by deflecting and bridging the crack,and pinning the propagating crack tip.Furthermore,addition of Ag did not have an adverse effect on the critical current density of Bi2223 superconductors.(2) The friction coefficient of pure Bi2223 dropped abruptly when the temperature cooled below the superconducting transition temperature.In normal state the average friction coefficient was 0.4 and the friction coefficient changes in the range of about 0.4.At liquid nitrogen temperature,the friction coefficient decreased sharply to 0.2, and the friction coefficient changes in the range of about 0.15.(3) Serf-lubricating composite Bi2223-Ag had fine tribological property.In normal state,the friction coefficient of composites was 0.15 and the wear rate was 10×10-4m3(N·m)-1 when the Ag content was 5wt.%.the friction coefficient of composites was 0.16 and the wear rate was 4.5×10-4 m3(N·m)-1 when the Ag content was 10wt.%.(4) At vacuum,the friction coefficient decreased with the increase of Ag content,and Ag content is between 5%-15%,the friction coefficient changed little, for about 0.15.When the Ag content to more than 15%,the friction coefficient was significant increase.(5) From 20℃—500℃,The friction coefficient of pure Bi2223 with temperature showed an upward trend in the increase in.However,When the Ag-doped Bi2223,the friction coefficient with the more complicated the relationship between temperature.The friction coefficient of composite materials was minimum at 300℃and when the Ag content of 20%,the high temperature friction of composite materials was better.
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