| A.Dayo and J·Krim had studied the interface friction of Pb surface and nitrogen molecule adsorbed on it with quartz crystal microbalance(QCM),and they observed the friction dropped abruptly when the temperature was cooled to the superconducting transition temperature(TC=7.2K) of Pb,and they proposed the friction originated from the electron and phonon.They associated macroscopic friction with atomic structure at first.The low temperature friction has very strong practical background, such as the superconducting device,the spatial detector,the liquid hydrogen pump and the liquid oxygen pump bearing work under low temperature.The superconducting transition temperature of pure metal is very low(in liquid helium), there is no utility value.High temperature superconducting materials YBa2Cu3O7-δ(YBCO) and Bi2Sr2Can-1CunO2n+4,n=2,3(BSCCO) can generate superconducting transition at liquid nitrogen temperature,the cost is reduced greatly. It has strong theory and utility value to study the tribological properties of high temperature superconducting material.The prime work is as follows:1.The superconducting material YBCO powders were prepared by the sol-gel method.The PbO/YBCO and Ag/YBCO composite samples were prepared by doping PbO and Ag in YBCO.The phase and microstructure of the composite samples were studied by XRD,SEM and TEM.Mechanical and superconducting properties of the composite samples were tested and the tribological properties were investigated in low temperature,ambient temperature,vacuum and high temperature.2.The friction coefficient of YBCO against stainless steel is 0.40～0.60 at ambient temperature.The friction coefficient decreases to 0.20～0.25 abruptly,finally keeping under 0.20 stably when the temperature is below superconducting transition temperature of the YBCO.In the sintering process of preparing PbO/YBCO composites,PbO reacts with few YBCO,producing BaPbO3 oxide compound, leading to the strengthening of grain boundaries of YBCO and improving the density, the hardness and the anti-wear performances of the PbO/YBCO composite.3.The XRD,SEM and TEM results show that the Ag particles are uniformly distributed in the YBCO matrix,improving the density and the fracture toughness of YBCO.The tribological test results show the friction coefficient of the Ag/YBCO composite is 0.20～0.30.The friction coefficient of 10%Ag/YBCO is 0.20,the worn rate is 8.96×10-5mm3·(N·m)-1.The friction coefficient 10%Ag/YBCO is 0.10～0.20 at 100～500℃.The friction coefficient of YBCO is 0.40～0.60 in vacuum.The vacuum does not have obvious effect on the reduction of friction coefficients.4.The Bi2212 and Bi2223 powders were prepared by solid state reaction method, then with different mass fraction Ag was doped in Bi2212 and Bi2223,composite samples were prepared by powder metallurgy method.The phase and microstructure of the composite samples were studied by XRD,SEM and TEM.The tribological test results show that the friction coefficient of Bi2212 against stainless steel is 0.30～0.40 at ambient temperature and decreases abruptly to 0.11 when the temperature is cooled about liquid temperature(T＜TC).The friction coefficient of Bi2223 against stainless steel is 0.30～0.50 at ambient temperature and decreases to 0.17 when the temperature is cooled about liquid temperature(T＜TC).At ambient temperature, normal load and slid velocity,the friction coefficient of 10%Ag/Bi2212 against stainless steel shows lower value 0.20 and the wear rate of 15%Ag/Bi2212 is minimum 9.5×10-5mm3·(N·m)-1,the friction coefficient of 20%Ag/Bi2223 against stainless steel is 0.20～0.35 and the wear rate of 5%Ag/Bi2223 is minimum 1.78×10-5mm3·(N·m)-1.5.The electron contribution to friction is observed in macroscopic friction experiment.The tribological mechanism of the superconductor is discussed.Ag doped in superconductivity ceramics,Ag particles may resist crack propagation by deflecting and bridging the crack,and pinning the propagating crack tip.The Ag film forms on the surfaces of superconducting composites during friction process and conducts friction heat promptly.The lubrication of soft metallic film and loading of hard matrix is the mechanism of decreased friction and anti-wear of composites...
|
PDF Full Text Request