Tribological Properties Of High Temperature Superconducting Material YBa₂Cu₃O₇ And Its Composites

With the development of technology of universe space, the lubricating desire of space machinery was improved gradually. 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. Therefore, it is very important and urgent to research and develop new friction material which has outstanding synthetical function.In the aspect of mechanism, 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 (T_c=7.2K) of Pb, and reported the electronic contribution to friction. 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 practicality, the superconducting transition temperature of pure metal is very low (in liquid helium), and is hard to apply. However, the superconducting transition temperature of high temperature superconducting material YBa₂Cu₃O_7-δ (YBCO) is above 90K, which can be realized at liquid nitrogen temperature. And it has the layer structure similar with solid lubricating materials such as graphite and MoS2. So, we chose it to study.YBCO belongs to ceramics, and its brittlenesss is large. The weakness is hard to machining. Recently, several studies have reported the effect of Ag addition on mechanical and superconducting properties of YBCO superconductors. Addition of appropriate amounts of Ag as a second phase to YBCO superconductors was reported to improve the fracture toughness of YBCO and not to have an adverse effect on the superconductivity of YBCO. However, its tribological property was studied little.This study was using powder metallurgy method to prepare YBCO-Ag self-lubricating composite with uniform and compact structure. The microstructure was studied by XRD and SEM. Ag was doping in two routes, metal Ag powder and AgNO₃ crystal. 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 YBCO 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 YBCO superconductors.(2) The friction coefficient of pure YBCO dropped abruptly when the temperature cooled below the superconducting transition temperature. In normal state the average friction coefficient was 0.4, and at liquid nitrogen temperature, the friction coefficient decreased sharply to 0.15.(3) Self-lubricating composite YBCO-Ag had fine tribological property. In normal state, the tribological property of composites which prepared from Ag was best when the Ag content was 10wt.%. However, the tribological property of composites which prepared from AgNO₃ was best when the Ag content was 5wt.%. At vacuum, the friction coefficient decreased with the increase of Ag content, and didn't change with the sliding velocity. The friction coefficient of pure YBCO decreased with the increase of temperature from normal to high temperature, and the friction coefficient of composite is little influenced by temperature When YBCO was doped with Ag.(4) Due to the finer and better dispersed Ag particles in matrix, the composites prepared from AgNO₃ show the better combined properties than the composites prepared from Ag.