| Since the development of modern science,the magnetic materials and the electromagnetic technology,the electromagnetic equipment is applied widely.The magnetic field can induce the variations of mechanical,physical and chemical properties of the magnetic materials,which may affect the tribological properties and useful life of the friction pairs in the electromagnetic equipment.This thesis mainly focuses on the coupling mechanism of metal materials during dry friction in DC magnetic field and the main elements and laws for the effects of magnetic field on the tribological properties of the materials.The theories of dry sliding tribology in the magnetic field is enriched,meanwhile,the theoretical basis is provided to the practical application of dry tribology in magnetic field and the active control of the tribological system.By using the HY-100 pin-disk tribometer,the friction-wear tests of different magnetic materials with or without magnetic field had been conducted.The rule for the influence of magnetic induction intensity,velocity and load on the 45 steel tribological properties had been studied.The morphological and ingredients variations of the wear tracks on the pin-disk and the debris during the severe-mild wear transforming process of the 45 steel at low velocities has been analyzed.Depending on the electromagnetic theories,the 3D static FEM(finite element model)of pin-disk friction pairs in magnetic field,the 2D static contact FEM of asperities in magnetic field,the force model of debris and the physical model for severe-mild wear transition of 45 steel promoted by the DC magnetic field were built.Through the tests and analysis above,following conclusions can be obtained:(1)The magnetic property of material and friction contact condition of the pin and disk specimen can affect the distribution of magnetic field in friction contact area.In addition,the distribution of magnetic field can influence the tribological properties of materials by affecting the magnetic attraction force between pin and disk,the distribution in friction contact area and the tribological behavior of the debris,as well as the electromagnetic induction phenomenon and the dynamic magnetization phenomenon,which are generated during the relative friction movement.(2)The effects of DC magnetic field on the tribological properties of materials with different magnetic properties is related to the magnetization characteristic of the material.For the material with larger permeability,its magnetization effect is more significant,the coupling effect between external magnetic field and it is greater,the magnetic induction in the friction contact area is bigger and the effect of the magnetic induction on it is more dramatic.At low velocities,the external magnetic field has little effect on the tribological properties of diamagnetic materials(such as red copper);however,the external magnetic field has a certain effect on that of paramagnetic materials(such as Ti alloy),since it reduces the adhesive wear proportion and increases the percentages of abrasive wear and oxidation wear.The magnetic field affects the tribological properties of ferromagnetic material 45 steel more dramatically,since it forms an oxidation film on the wear surface,decreases the wear coefficient prominently and enhances the friction noise.Therefore,the magnetic field is suitable to change the tribological properties of ferromagnetic materials.(3)The effects of magnetic induction intensity on the tribological properties of 45 steel is closely related to the magnetization curve.At low velocities,the increase of magnetic induction intensity can decrease the wear coefficient of 45 steel;meanwhile,it also can rise the temperature increment of pin and the friction noisy.When the magnetic induction intensity is smaller than the threshold value,the main wear mechanism is abrasive wear and adhesion wear.When the magnetic induction intensity is bigger than the threshold value,the wear mechanism turns to oxidative wear.Thus,the transforming of severe-mild wear is implemented,during which the wear coefficient is decreased dramatically and the friction noisy is raised.Since the decreasing trend of the wear coefficient slows down with the increase of magnetic induction intensity,the applied magnetic field need not too big when it is used to reduce the wear of ferromagnetism material.(4)In different magnetic induction intensity,the increase of load can reduce the friction coefficient and enlarge the temperature increment of the pin,but it has little effect on the wear mechanism of the 45 steel.When the load is smaller,the magnetic field can decrease the wear coefficient,meanwhile it can induce louder friction noisy.When the load is larger,the magnetic field can decrease the friction coefficient,meanwhile it also can decrease the wear coefficient and the friction noisy.When the magnetic induction intensity is smaller,the increase of load can decrease the wear coefficient and increase the friction noisy.When the magnetic induction intensity is larger and the oxidative wear is formed,since the increase of load can destroy the oxidation film and increase the percentage of abrasive wear,the friction coefficient and friction noisy are decreased,however,the wear coefficient has little change.When both magnetic induction intensity and load increase,the friction coefficient and the wear coefficient decrease but the temperature increment of pin increases.(5)The effects of magnetic field on the tribological properties of 45 steel is closely related to the velocity.In different magnetic induction intensity,the increase of velocity decreases the friction coefficient and increases the temperature increment of pin.After applying the external magnetic field,the velocity range of oxidative wear of 45 steel has been expanded.At low velocity,the magnetic field promotes the oxidative wear of 45 steel and decreases the wear coefficient dramatically.At medium or high velocity,as the increase of the velocity,the number of debris,which are attracted by the magnetic field is decreasing,thus the magnetic field has little effect on the wear mechanism of 45 steel,friction coefficient and the wear coefficient.When both magnetic induction intensity and velocity increase,the friction coefficient and the wear coefficient decrease,but the temperature increment of pin increases.Therefore,the effectiveness of using the magnetic field to reduce the wear is better at low velocity.(6)The dry sliding tribological properties of material is determined by thickness,width,property,and constituent of the capture layer,which is formed by the debris attracted by magnetic field.The formation,thickness and width of the capture layer are affected by the debris size,magnetic property,velocity,magnetic induction intensity,the distribution of magnetic induction intensity,etc.When the debris size is smaller,the magnetic permeability of debris and the remanence are greater,the velocity is lower and the magnetic induction intensity is larger,the debris will be attracted to the wear tracks more easily to form the capture layer.(7)The larger magnetic field intensity gradient in the gap between the asperities is favorable for the oxygen molecule gathering on the contact point of the asperity,thus the oxidation in this area is promoted.The oxidation of wear tracks and debris is promoted by the active effect and the thermal effect of dynamic magnetizing induced by the magnetic field,the mechanical effect during friction process,the oxidation accumulation effect led by the debris attracted by magnetic field participating in the friction repeatedly,the refining of debris by the magnetic field,etc.(8)The debris,which is attracted to the friction contact area and the wear tracks on the disk by magnetic field,can forms a capture layer with a certain thickness and width,and participates in friction repeatedly.Therefore it promotes the oxidation of wear tracks and debris of ferromagnetism material 45 steel,meanwhile,it also promotes the wear tracks surface hardening and the debris refining.When the oxidation of wear tracks and debris as well as the debris refining have reached a certain degree,a compact oxide film,which can promotes the transforming of severe-mild wear and enhances the wear resistant of 45 steel,will be formed on the wear surface. |
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