| As a new and modern urban rail transit,the development of maglev trains has become the primary plan of large and medium-sized cities in China.Due to that the current technology of maglev train is not mature enough,there are some problems in vehicle grounding and line grounding,which makes the brake pad of maglev train suffer current during service.The service life of the brake is greatly reduced because of the current,which not only increases the loss of materials,but also poses a threat to the life safety of passengers.Therefore,this thesis focuses on the friction and wear behavior of copper-based powder metallurgy,the current medium-low speed maglev brake material in service,and in-depth study of the friction and wear characteristics with electrical current of the copper-based powder metallurgy/Q235-B friction pair;At the same time,the friction and wear tests of carbon-ceramic,another type of brake material for maglev trains,were carried out under the same working conditions to compare the friction and wear properties between the two materials,in order to provide theoretical reference for the selection of brake material in the future.In this thesis,the existing rail grinding test equipment was independently improved,so as to complete the current-carrying friction and wear tests of two friction pairs(copper-based powder metallurgy/Q235 B and carbon-ceramic/Q235B).Through the analysis of the friction coefficient and wear rate during the test,and combined with the stereo microscope(SM),scanning electron microscope(SEM),white light interferometer,and electronic energy spectroscopy(EDS)to characterize the wear morphology of the brake pad material.The effects of contact load,sliding speed,current and dry-wet environment on the friction and wear properties of copper-based powder metallurgy/Q235 B friction pair were studied from the perspective of basic tribology.The difference in friction and wear performance of copper-based powder metallurgy/Q235 B and carbon-ceramic/Q235 B friction pairs under the same parameters was compared and studied.The conclusions are as follows:(1)When there is no current,the friction coefficient and wear rate of copper-based powder metallurgy/Q235 B friction pair will decrease with the increase of sliding speed,and increase with the increase of contact load.When the sliding speed increases,the granular debris on the surface of copper-based powder metallurgy will be compacted to form a dense continuous third body,which fully plays the role of friction reduction and wear resistance.When the contact load increases,the adhesion wear and abrasive wear on the surface of copper-based powder metallurgy will be aggravated.(2)When carrying current,the friction coefficient of copper-based powder metallurgy/Q235 B friction pair will decrease with the increase of sliding speed,while the wear rate of copper-based powder metallurgy will increase with the increase of sliding speed.Mechanical wear zone and arc ablation zone appear on the surface of copper-based powder metallurgy under current.With the increase of sliding speed,the area of ablation zone and the adhesion pits will increase gradually,resulting in a large number of flake debris.At the same time,the friction coefficient of the copper-based powder metallurgy/Q235 B friction pair will increase with the increase of the contact load,and the wear rate of the copper-based powder metallurgy will show a "U"-shaped change trend with the increase of the contact load.The main wear mechanism of materials under low load is electrical wear.With the increase of load,the wear mechanism will gradually change from electrical wear to abrasive wear,fatigue wear and adhesive wear(3)The friction coefficient of copper-based powder metallurgy/Q235 B friction pair under current is lower than that without current,and decreases with the increase of current.The wear rate of the copper-based powder metallurgy increases with the increase of the current,and is higher than that without current.The cutting action of single hard particles in the matrix is intensified by the current intervention,leaving wide and deep furrows on the surface of the matrix.With the increase of current,the arc ablation area will gradually increase.(4)In the absence of current,the friction coefficient of the copper-based powder metallurgy/Q235 B friction pair and the wear rate of copper-based powder metallurgy under wet environment are much smaller than those under dry environment,and both will decrease with the increase of rainfall.Due to the isolation and erosion of water,it is difficult to completely form the third body on the material surface.Under current-carrying conditions,compared with dry environment,the friction coefficient of copper-based powder metallurgy/Q235 B friction pair has an upward trend in the light rain stage,but as the rainfall further increases,the friction coefficient begins to gradually decrease.The wear rate of copper-based powder metallurgy decreases with the increase of rainfall,and is generally lower than that under dry environment.The intervention of water will intensify the effect of arc erosion,accompanied by a large black focal ablation pit at the tail of arc ablation zone.(5)In the absence of current,the friction coefficient of carbon-ceramic/Q235 B friction pair is higher than that of copper-based powder metallurgy/Q235 B friction pair.While the wear rate of carbon-ceramic material is much lower than that of copper-based powder metallurgy material.When there is current,the friction coefficient of the carbon-ceramic/Q235 B friction pair decreases with the increase of the current,and it is gradually smaller than that of the copper-based powder metallurgy/Q235 B friction pair.The wear rate of carbon ceramics increases greatly with the increase of current,and is always higher than that of copper-based powder metallurgy materials.The wear surface of carbon-ceramic under current has obvious plastic deformation,electrical wear and adhesive wear are very severe,and the material loss is more serious with the increase of current.Therefore,from the friction coefficient and wear rate of the two brake materials,the friction and wear properties of carbon-ceramics under non-current-carrying conditions are better than those of copper-based powder metallurgy.The friction and wear properties of copper-based powder metallurgy are better than those of carbon ceramics under current-carrying conditions. |
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