Composition Ratio And Friction And Wear Properties Of Cu-matrix Powder Metallurgy Friction Materials For High-speed Trains

Copper-based powder metallurgy material is the most commonly used brake pad material for high-speed railway trains.Its excellent friction performance mainly comes from the synergies of copper matrix,reinforcing,friction and lubrication components.With the increasing speed,the brake pad will be subjected to higher pressure,shear force and thermal load.In order to prepare high-performance brake pads to adapt to severer braking conditions,it is necessary to systematically analyze and study the action mechanism of each component in the braking process and its influence on the evolution process of friction film.In this paper,the composition of friction materials was regulated through component design.The wear surface was characterized by scaled pin-disk experiments,combined with microhardness tester,scanning electron microscope,energy spectrometer and laser confocal microscope.The effects of each component on the friction and wear properties of materials were studied,which laid the foundation for successfully preparing powder metallurgy brake pads meeting the braking requirements of high-speed railway trains.,The main contents of this thesis are as following:The effects of the type and content of iron powder on the friction and wear properties of copper matrix composites were studied.The results show that the wettability between ferrophosphorus and matrix is poor,and carbonyl iron is more suitable for strengthening matrix.The increase of iron content is conducive to the formation of friction film,but excessive iron powder will produce a large amount of iron oxide,which makes the friction film loose and easy to peel off.The increase of sliding speed will promote the escape of materials,making it difficult to form a thick friction film on the friction surface.The faster the sliding speed is,the higher the stress is,the more likely the friction film is to crack,resulting in the rapid increase of wear.The effects of graphite type and content on the friction and wear properties of copper matrix composites were studied.The results show that the proper introduction of colloidal graphite has a significant effect on the microhardness and plastic deformation resistance of the friction film.Colloidal graphite is easy to react with iron to form cementite.Cementite has high strength,and the subsurface cementite plays a strong supporting role on the friction surface.At the same time,cementite participates in the formation of friction film and enhances the strength of friction film.However,too much colloidal graphite destroys the continuity of the matrix.The results show that when the content of colloidal graphite is 1% and the content of copper plated graphite is 19%,it has better tribological properties under 350 km/h high-speed braking.The effects of ferrochromium type and content on the friction and wear properties of copper matrix composites were studied.It is found that the friction material containing coarse ferrochrome is seriously worn after the braking speed exceeds 250 km/h.The hardness of friction materials decreases with the increase of coarse ferrochrome content.The matrix hardness of friction materials only containing fine ferrochrome reaches 16.95 HBW,while the matrix hardness of friction materials only containing coarse ferrochrome is only 10.72 HBW.Friction materials only containing fine ferrochrome can make the friction coefficient more stable.In general,10% fine ferrochrome should be selected as the friction component.In this thesis,different friction material samples were fabricated according to the powder metallurgy process and the research on the composition(reinforcement,lubrication and friction components)and proportion of high-speed train braking friction materials.The small samples were tested to investigate the wear resistance,friction coefficient and stability under the simulated high-speed railway braking working conditions.And the bonding interface of the components was microscopically observed to select the formula of braking friction materials for high-speed trains with the best comprehensive performance.It has guiding significance for the preparation of high-speed railway brake pads that meet the requirements.