| The system consisting of pantograph and contract wire is the core of energy supply to ensure the safe and efficient operation of electric locomotive.However,with the continuous innovation and development of the railway science,the technological breakthrough of high-speed and heavy-loading trains presents new challenges to the pantograph and overhead contact wire system: it is required that the pantograph and overhead contact wire system can not only provide more electric energy for the traction or braking behavior of the train in service,but also better deal with the deterioration of the service environment under the condition of high-speed and heavy loading,so as to ensure that the quality of current receiving of the train will not be affected.It also means that the current-carrying friction pairs in the pantograph and overhead contact wire system need to further improved their conductivity,wear resistance,so that they can prolong their service life.Though it has such drawbacks like low hardness,poor wear resistance and thermal softening properties which limit its application to some extent,the copper contact wire also has excellent electrical conductivity,high plasticity and toughness,good machinability and low preparation cost,which has great potential development value.Therefore,in this paper,a gradient nanostructured layer with excellent properties was prepared on the surface of copper bar by surface mechanical rolling treatment(SMRT)technology,which was characterized by optical microscope OM,transmission electron microscope TEM and electron backscattering diffraction EBSD.The results show that the grain size of the surface layer of the processed sample is obviously refined,and the grain size is about 200～300 nm and the thickness of the nanocrystalline layer is about 10～15 μm,and the gradient characteristics are along the core direction.The near surface hardness is increased to about 1.53 times of the matrix hardness,and there is a layer of hardness influence layer with thickness of about 1800 μm.The nanomechanical properties and elastic modulus of the surface layer are significantly improved,which has certain creep resistance.Subsequently,a self-designed current-carrying friction system was used to carry out current-carrying tribological tests under different current-carrying factors(load current,contact load and sliding speed),and the current-carrying tribological behavior of rolled copper samples was studied.The results showed that:(1)The increase of load current will lead to the increase of electrical adhesion between the friction interfaces.The friction coefficient of the untreated copper sample increases linearly,and the wear adhesion intensifies.The friction mechanism changes from abrasive wear to abrasive wear,and fatigue wear and adhesive wear work together.The friction coefficient of the surface rolling treated sample decreased in a step-like manner and was at a relatively low level.The contact resistance during the current-carrying friction process was reduced by about 21.6 % on average compared with that of the untreated sample,and the cumulative arc energy was reduced by about 27.44 %.The wear mechanism changed from adhesive wear to adhesive wear,abrasive wear and fatigue wear.(2)The increase of friction load will lead to the increase of mechanical friction at the contact interface,and the ploughing phenomenon on the friction surface is obvious.The contact resistance and cumulative arc energy are reduced.The friction coefficient of the untreated copper sample is greatly reduced,and the surface damage is serious.The damage mechanism is changed from adhesive wear to abrasive wear and adhesive wear.Although the friction coefficient of rolled copper samples increased slightly,it was still less than that of untreated samples.The damage mechanism changed from adhesive friction to adhesive wear and abrasive wear,and the surface damage was relatively slight.(3)The change of sliding velocity will lead to the coupling effect of mechanical friction and electrical erosion in the current-carrying interface.The arc ablation oxidation phenomenon on the contact surface of the untreated copper sample leads to an abnormal increase in the friction coefficient and a high level,and the damage of the friction surface is serious.The sputtering phenomenon can be observed on the graphite friction surface,and the wear mechanism is gradually changed from abrasive wear to adhesive wear,supplemented by electrical oxidation wear and adhesive wear,supplemented by abrasive wear.After surface rolling processing,the copper sample showed a more stable and lower friction coefficient and contact resistance under the increasing friction shear.The wear process was less affected by the friction,and the wear mechanism was the combined action of abrasive wear and adhesive wear. |
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