Study On The Paths Of Impact Of Magnetic Field On The Dry Friction Of Medium Carbon Steel And Their Relevance

With the development of electro-magnetic technology,a variety of electromagnetic equipments have been widely used,in which the friction pairs work in the the presence of magnetic field.The magnetic field can not only change the mechanical and physical-chemical properties of the ferromagnetic materials,but also alter the contact behavior between the friction surface of the friction pairs,which the tribological characteristics and service life of the friction pairs in electro-magnetic equipment are affected.The ways of influence of external magnetic field on the dry sliding friction and wear characteristics of medium carbon steel and their relevance were investigated in this paper.The key factors and regularities that magnetic field affects the tribological properties of materials were explored.This may help to enrich the contents of dry tribology and provide theoretical basis for the practical application of dry tribology under magnetic field and the active control of friction system.The friction and wear tests of medium carbon steel with and without magnetic field were carried out by using self-made pin-and-disc friction tester.Aiming at the behavior of wear debris and its function in the magnetic-friction process,the effects of magnetic field on the tribological properties of medium carbon steel under various conditions(the wear debris capture is interfered and the oxygen content changes in environmental atmosphere)were studied.The morphology and composition of wear debris and friction surface during friction were analyzed.The ways of influence of external magnetic field on the friction and wear characteristics of medium carbon steel and their relevance were clarified.On the above foundation,the wear debris behavior model and the formation model of the oxide modified layer on friction surface were established.The magnetic field has a significant effect on the friction and wear characteristics of medium carbon steel through two ways.When there is no magnetic field,the pin specimen has been in the state of severe wear during friction.The wear type of friction pairs is mainly abrasive wear and adhesive wear.The wear debris on the friction zone and the friction surface of the disk specimen is less and evenly distributed.Under the action of magnetic field(8.7k A/m),the friction process can be divided into such three stages as cutting transition stage,grinding stage and magnetic-friction stabilization stage.In the grinding stage,the wear debris became gradually small and oxidized,and the wear debris capture layer formed on the friction zone and the friction surface of disk specimen.And then the severe-mild wear transition occurred.In the magneticfriction stabilization stage,the oxide modified layer formed on the friction surface and the oxidation wear occurred between the friction pairs.The wear reduction rate of magnetic field for medium carbon steel can reach more than 90%,and its influence ways mainly include such two kinds of wear debris capture layer in grinding stage and oxide modified layer in magnetic-friction stable stage.The extent of influence of magnetic field on the friction and wear characteristics of medium carbon steel friction pairs have do with its intensity.With the increasing of magnetic field intensity,the wear of pin specimen decreased gradually and reached stability.When the magnetic field intensity is 2.1k A/m,the pin specimen has been in a state of severe wear during friction.The wear debris on the friction zone and the friction surface of disk specimen was less and evenly distributed.No magnetic-friction stable state reached,and the abrasive wear occurred between the friction pairs.When the magnetic field intensity is greater than 4.3k A/m,the severe-slight wear transition began to occur.The wear debris capture layer was formed on the friction zone and the friction surface of the disk specimen.The capture wear debris filled up and was higher than the grooves on the frction zone.The oxide modified layer was formed on the friction surface of pin specimen,and the oxidation wear occurred between the friction pairs.With the increasing of the magnetic field intensity,the width of the wear debris capture layer increased gradually,and the earlier the severe-mild wear transition occurred,the faster the magnetic-friction stable state reached.When the wear debris capture is disturbed,the regularities of influence of magnetic field on the friction and wear characteristics of medium carbon steel friction pairs change significantly.The residual debris on the friction surface of the disc specimen significantly reduced when the removal wear debris device was adopted during friction.Compared with the magnetic friction of medium carbon steel under normal conditions,the wear amount of pin specimen increased significantly under magnetic field of the same intensity,and no severe-mild wear transition occurred in the friction process.No oxide modified layer formed,and the abrasive wear occurred between the friction pairs during friction.The influence of magnetic field on the friction and wear characteristics of medium carbon steel was independent of the magnetic field intensity when the wear debris capture was disturbed.The magnetic field has a certain influence on the friction and wear characteristics of medium carbon steel pin/302 steel disk friction pairs.However the influence regularity is different from that of medium carbon stee friction pairs.When the magnetic field intensity is 17.4k A/m,the pin speciemn was in a serious wear state,and no severe-mild wear transition occurred in the friction process.The wear extent of the pin specimen was greater than that without magnetic field.The less wear debris remained on the friction zoneand friction surface of the disk spenimen.There is no oxide modified layer on the friction surface,and the abrasive wear occurred betweeen the friction pairs.The oxygen content in the friction atmosphere has a certain influence on the friction and wear characteristics of medium carbon steel friction pairs.When there is no magnetic field and the oxygen content of friction atmosphere is 0.2%-21%,with the increasing of oxygen content,the oxidation degree of wear debris increases slightly,and the wear amount of pin specimen increased.The wear mechanism transformed from adhesive wear(0.2% oxygen content)to abrasive wear.When the oxygen content is 100%,and the oxidation degree of wear debris was higher.The wear of pin specimen decreased significantly.The severe-slight wear transition arose,and the oxidation wear occurred between the friction pairs.When the oxygen content of friction atmosphere is 0.2% in the presence of magnetic field(8.7k A/m),the oxidation degree of wear debris was low,and no severe-mild wear transition occurred.The adhesive wear occurred between the friction pairs.When the oxygen content of the friction atmosphere is more than 0.5%,the oxidation degree of the wear debris is relatively high.In the process of friction,the grinding stage and the magnetic-friction stable stage occurred,and severe-slight wear transition arose.The oxidation wear occurred between the friction pairs.Moreover,with the increasing of oxidation degree of the wear debris,the occurrence time of grinding stage and that of severe-mild wear transition advanced.The capture wear debris plays an important role in maintaining the dynamic balance of the magnetic-friction stable state.During the friction process of the friction pairs of medium carbon steel in the magnetic-friction stable state without magnetic field,there was almost no residual wear debris on the friction zone and friction surface of the disk specimen.The oxide modified layer on the friction surface of the pin specimen gradually disappeared and the contrast of pin friction surface appeared metallic-bright white.The anti-wear time of the oxide modified layer was about 400 s,and then the mild-severe wear transition occurred.It is the premise and foundation for anti-wear of medium carbon steel and achievement of the magnetic-friction stable state owing to magnetic field that a debris capture layer is formed on friction area and the wear debris is oxidized to approprite certain extent.The wear debris movement during friction of medium carbon steel under the appropriate magnetic field can be divided into four continuous stages: formation and adsorption,movement with disc specimen,grinding and joining.In the process of wear debris movement,such physical and chemical changes as refinement,oxidation and agglomeration take place gradually.The magnetic field intensity,the size and composition of wear debris and material of disc specimen have important influence on the wear debris capture.The oxide modified layer can be formed in the discontinuous groove by metallurgical bonding between the wear debris and groove,and among the wear debris.The subsurface layer of the pin specimen caused by capture wear debris layer is mainly composed of deformed pearlite in the magneticfriction process,which can improve the anti-wear of the pin specimen to a certain extent.