Study On Preparation And Performance Of Layer Structure Pantograph Contact Strip

Pantograph contact strip is the main collecting electricity component of electric locomotive. It can help electric locomotive power supply system to obtain electric energy. The quality of contact strip has important impact on current carrying condition of electric locomotives and service life of contact wire. According to its material, strip can be classified into three types:carbon series strip, metal series strip and composites strip. As operation speed of electric locomotive continuous improvements, traditional contact strip can not meet the demand of the high speed train. In this paper, we designed a new structure contact strip to overcome the shortcomings of traditional contact strip. The contact strips obtained excellent conductivity by laminar laying copper mesh being framework, and had good wear resistance due to composite laminated structures. Focus on development of layer structure pantograph contact strip, the structure design, formula and process optimization were investigated in this paper. We could get the optimal formula and process condition by comparing resistivity, impact strength and wear resistance of different formula and process samples. The conductive and wear mechanism of contact strip were also analyzed.In the structure design of pantograph contact strip, layer structure was determined by balancing against the influence of composites component and copper mesh on resistivity, impact strength and wear property. The optimal pore diameter size of copper mesh was3.16mm×6.3mm and the thickness was0.48mm.In the materials composition design of layer structure resin matrix pantograph contact strip, influence of the resin content and fiber type on performance of contact strip was analyzed. The results showed that phenolic resin content had small impact on the resistivity of contact strip. The impact strength decreases with the increase of phenolic resin content. When phenolic resin content was lower, there was a certain angle between impact fracture section and impact load direction, which suggested that fiber reinforced played an important role in changing directions of crack as well as slowing crack propagation, showing higher impact strength. Impact fracture section of pantograph slide plate was not only smooth but also on the same with load direction when phenolic resin content was higher, appearing brittle fracture with lower impact strength. With the increase of phenolic resin content, the wear loss increased first and then decreased. The wear loss was lowest when resin content was26.4%. Thus, the optimal content of phenolic resin was26.4%. The study of electrical sliding wear phenomena and behavior of composite contact strip revealed that adding carbon fiber could facilitat conductivity and wear resistance. The existence of copper fiber obviously enhanced the conductivity and improved the arc ablation phenomenon. The wear resistance was greatly improved due to extinguishing of the wollastonite fiber. So we choose carbon fiber, copper fiber and wollastonite fiber as hybrid fiber to reinforce contact strip.The uniform design and regression analysis method were used in formula design of contact strip. The influence of carbon fiber, graphite, wollastonite fiber, copper powder and copper fiber content on properties of pantograph contact strip was investigated. The appropriate formula was determined by mean of regression analysis. The results showed that electrical conductivity was linear to copper fiber content and decreased with the increase of copper fiber content. Impact strength of pantograph contact strip was in direct proportion to CF content. The friction coefficient of pantograph contact strip was mainly influenced by carbon fiber, graphite and wollastonite content. Many factors including CF, graphite, wollastonite and copper content influenced wear loss, and significant interaction was found in CF, graphite and wollastonite fiber. Copper powder had smaller effect to performance of pantograph contact strip. According to the results of uniform design and regression analysis experiment, the formula of contact strip was optimized and the optimum of layered structure resin matrix pantograph contact strip as follows:phenolic resin at26.4%, nitride rubber at2.4%, carbon fiber at15.2%, graphite at14.2%, wollastonite fiber at4.8%, copper fiber at8%, copper powder at5%, and copper mesh at24%.By the orthogonal experiment, the mold pressing process was optimized. Based on test results and range value, the sequence of affecting contact strip behavior was ranked as pressure applying time>molding temperature>holdup time> pressure. The optimum process parameters were molding temperature at170℃, holdup time at5min·mm-1, pressure at60MPa, and pressure applying time at3min.Raman spectroscopy (Raman), X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) were used to investigate the evolution of surface structure and morphology before and after oxidation treated. Thereby the best oxidation time was determined. Carbon fibers oxidated by nitric acid was treated by silane coupling agents and investigated the effect of different surface treatment on performance of contact strip. The results indicated that nitric acid oxidation treatment did not change the entity structure of carbon fibers. With the increase of the oxidation time, interplanar spacing of carbon fiber surface increased and crystalline size decreased slightly. The entire structure disordered and R values of carbon fiber increased with the increase of oxidation time. The grooves of surface became wider and deeper with the increase of the oxidation time. Some big holes and grooves could be observed when the oxidation time reached to12h, which would severely damage the fiber. After oxidation treatment to9h,-COOH,-OH could be found by FTIR. So the best oxidation time was9h. After silane coupling agents treated, the grooves of the surface were covered with thin films and the surface grooves became shallower compared with the fiber treated by nitric acid. The results showed that silane coupling agents were effective for overlaying the micro cracks and grooves. The performance of contact strip which reinforced by different trcated carbon fiber was investigated. Surface treatment had little effect on resistivity of contact strip. The resistivity of contact strip increased slightly after silane coupling agent treatment. Impact strength increased after surface treatment by nitric acid and the impact strength increase by37%after silane coupling agent treatment. From scanning electron microscopy images, it could be seen that the surface of fiber coated with resin in uneven thickness. The existence of flexible groups came from silane coupling agent facilitated absorption of impact energy, which indicated that silane treatment improved interface bonding strength between resin and fiber. The wear resistance was enhanced by surface treatment of carbon fibers, and the wear loss decreased by32%after silane coupling agent treatment. The technology of silane coupling agent compound treatment was an effective method to improve performance of contact strip.In order to improve heat resistance, we adopted calcinations to treat contact strip. The influence of calcinations temperature on properties of contact strip was analyzed to confirm the optimum temperature at600℃. The densification process was used to improve the properties of contact strip treated with calcinations. The results showed that the density, conductivity, impact strength and wear resistance were improved with increase of calcinations-impregnation times compared with contact strip treated by one-shot calcinations. The performance of contact strip after densification process was still lower than untreated contact strip. The existence of pore between fiber and resin restricted improvement of contact strip performance. The weight increase rate was decreased when increasing of impregnation time. The properties changed very slight when impregnation time was three, thus we confirmed the best densification process as four times of calcinations and three times of impregnation. The impact of resin content on performance of calcinations type pantograph contact strip was analyzed to determine optimal formula as follows:phenolic resin at25.8%, carbon fiber at15.2%, graphite at14.2%, wollastonite fiber at4.8%, copper fiber at8%, copper powder at8%, and copper mesh at24%.The morphologies of the worn surfaces were observed with SEM and EDS, and the wear mechanics were discussed. The results showed that the adhesive wear behavior changed from smearing type to scratches type after adding copper fiber and mesh. Silane compound treatment of carbon fiber would enhance adhesive strength between fiber and matrix to improve wear resistance. The wear mechanism of resin matrix contact strip was smearing type adhesive wear. Raising calcinations temperature made the major wear mechanism of contact strip changed from adhesive wear to adhesive wear together with abrasive wear.