【作者】 董丽荣；

【导师】 李长生；

【作者基本信息】 江苏大学 ， 材料学， 2009， 博士

【摘要】 八十年代末,A.Dayo和J.Krim等利用石英晶体微天平（QCM）研究金属Pb表面与在其上流动的氮气的界面摩擦,发现当温度降到Pb的超导转变温度时摩擦力突然大幅降低,提出了摩擦能量的耗散来源于电子和声子,把宏观的摩擦和原子结构联系起来。低温摩擦具有很强的实用背景,如超导装置、空间红外探测器、液氢泵和液氧泵轴承等都工作在低温下。纯金属要在液氦温度下才能实现超导转变,液氦昂贵价格,限制了其实际应用。而氧化物高温超导材料YBa₂Cu₃O_7-δ（YBCO）,Bi₂Sr₂Ca_n-1Cu_nO_2n+4 n=2,3（BSCCO）在液氮温度下可实现超导转变,研究其摩擦学性能具有理论意义和实用价值。本研究的主要工作如下:1.采用溶胶-凝胶法制备YBCO粉体,PbO和Ag以不同的质量分数分别添加到YBCO中,制备了PbO/YBCO和Ag/YBCO两种超导体复合材料,对其物相和显微组织进行表征,检测超导复合材料的力学性能和超导性能,分别研究了样品在低温、常温常压、真空及高温环境下的摩擦磨损性能。2.摩擦测试结果表明:室温下YBCO与不锈钢对摩,摩擦系数为0.40～0.60,平均为0.50,液氮温度下YBCO处于超导态（T＜T_C）,摩擦系数急剧下降,摩擦系数为0.20～0.25,最后稳定在0.20以下。PbO/YBCO复合材料在摩擦过程中,摩擦表面形成了转移膜,起到降低摩擦系数的作用。PbO在烧结过程中与少量的YBCO反应生成氧化物BaPbO₃,使晶粒之间的界面结合牢固,PbO/YBCO复合材料的密度、硬度、耐磨性能得到提高。3.XRD、SEM和TEM结果表明:Ag与YBCO不发生化学反应,Ag/YBCO复合材料密度和韧性得到提高。摩擦测试结果得到:Ag/YBCO摩擦系数为0.20～0.30,10%Ag/YBCO摩擦系数为0.20,磨损率为8.96×10^-5 mm³·（N·m）^-1。在100～500℃温度下,10%Ag/YBCO摩擦系数为0.10～0.20,真空下YBCO的摩擦系数为0.40～0.60,真空环境对摩擦系数没有明显的影响作用。4.采用固相反应法制备了Bi2212和Bi2223粉体,添加不同质量分数的Ag制备了Ag/Bi2212和Ag/Bi2223两种超导体复合材料。对样品的显微结构与力学性能进行了表征。摩擦测试结果表明:Bi2212与不锈钢对摩,摩擦系数为0.30～0.4平均为0.35,在液氮温度下Bi2212处在超导态,摩擦系数急剧下降,最后稳定在0.11。Bi2223与不锈钢对摩,摩擦系数为0.30～0.50,平均为0.40。在液氮温度下Bi2223处在超导态,摩擦系数下降到0.17。在常温、正常载荷和滑行速度下,10%Ag/Bi2212复合材料摩擦系数为0.20,15%Ag/Bi2212磨损率最低,为9.5×10^-5mm³·（N·m）^-1。20%Ag/Bi2223摩擦系数为0.20～0.35,5%Ag/Bi2223的磨损率最低,为1.78×10^-5mm³·（N·m）^-1。5.在宏观摩擦实验中印证了J.Krim等提出的电子摩擦理论,讨论了氧化物超导复合材料的摩擦机理。Ag添加到超导体中抑制裂纹萌生和扩展,在摩擦作用下向表面转移,在摩擦表面形成一层Ag转移膜,硬基底承载与软金属转移膜润滑作用使得超导复合材料表现出良好的减摩耐磨性能。更多还原

【Abstract】 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（T_C=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 YBa₂Cu₃O_7-δ（YBCO） and Bi₂Sr₂Ca_n-1Cu_nO_2n+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＜T_C）.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＜T_C）.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更多还原