Study On The Mechanism Of Friction And Wear With Electric Current

With the progress of science and technology, the development of the economy, and the expansion of exploration field, human being comes across a lot of peculiar working condition tribology problem. Power supply of high speed electric railway is a typical peculiar tribology problem; however the research concerning tribological characteristics of aluminum-stainless steel composite conductor rail and collector shoe used in subway third rail traction system has not been reported so far.In this paper, a new tester by modifying the jigs and the control system of conventional pin-on-disc apparatus has been developed. The friction and wear characteristics of aluminum-stainless steel composite conductor rail and collector shoe have been investigated on the tester with electrical current applied across the sliding interface. The morphologies of worn surfaces have been observed and analyzed by means of scanning electron microscope (SEM) and energy dispersive x-ray spectroscopy (EDX), X-ray diffraction (XRD), and surface profilometer, etc. The tribological characteristics of aluminum-stainless steel composite conductor rail and collector shoe under different electric current, sliding speed, polarity, and normal force have been tested systematically. Effects of bracing stiffness on the friction and wear with electrical current also have been investigated on the modified rig of CETR UMT-2 tester. The main obtained results and conclusions in this dissertation are as follows:1. The results show that the wear volume loss decreases with the increase of electric current and velocity. The current, normal force and sliding speed induce the temperature rises between the friction parts, and these are the key factors influence the friction and wear characteristics. The wear volume loss increases with the rising of temperature. The electrical wear loss has bearing on polarity. Water molecule adsorbed by sliding contact surface decomposed into hydrogen and oxygen ion under the influence of electrical field. The oxidation reaction occurred bewteen positive metal ions (or carbon ions) and negative oxygen ion weakened the bond bewteen the grains in the surface layer. The wear volume loss of a positive collector shoe is larger than that of a negative collector shoe. Surface profile analyse shows that wear volume losses of aluminum-stainless steel composite conductor rail increase with the increase of sliding speed, but the losses are under micron level, which are much less than wear volume losses of the collector shoe. With the increae of current, the friction cofficients fluctate under the less range, with the trend of first enhancing and then diminishing. the friction cofficients decrease with the increase of sliding speed, and nearly equal under different polarity.2,The friction cofficients increase with the increase of normal force under the current, but decrease with the increase of nomral force without current. With the increase of normal force, the electrical wear volume loss firstly decreases greatly then increases slightly in the shape of U. The experiment results show that a optimum normal force exists in friction and wear with electric current, the wear volume loss is least under the optimum normal force, and the optimum normal force increase with the increase of sliding speed. On the basis of tests and analyses, a three demension W-F-V (Wear volume loss-Normal force-Velocity) model are established. The model is fairly fitting of actual test results, and is valuable in the prediction of wear loss under current and the choice of optimum frictional condition. In the real working condtion of three rail, working under the the optimum normal force makes electrical power supply stable and wear loss much less, reduces maintenance frequency, ensures driving safety, and saves subway running cost.3,Mechanical wear and electric wear are major wear mechanism in the friction and wear between aluminum-stainless steel composite conductor rail and collector shoe. The mechanical wear includes adhesive wear, abrasive wear, oxidative wear, and fatigue wear in the tests. Mechanical wear and electric wear enhance mutually, and wear losses caused by electric arc erosion is much larger than those caused by mechanical wear. The wear process shows the results of being a synthetical effect by various mechanisms.4,Tests show that bracing stiffness of loading system has effect on the on the friction and wear with electrical current. Under different bracing stiffness, vibration amplitude of normal force increased with decreasing of the normal force, however, vibrations from the elastic system exhibited less than those from the rigid system. Spring's ratio of frictional energy to deformation energy has relation with the mass loss of the pair. If the ratio is larger, the wear loss is less. In working condition, selection of appropriate range of normal force and stiffness of spring is necessary to decrease the wear loss induced by arc erosion.5,Based on the three Axioms in Tribology, a system structure map, a system process map, and a electric circuit model are established. These results have established the analytical basis for research in friction and wear with electric current and are significant for the future research.