Design,Preparation And Friction Behavior Of Copper-Based Powder Metallurgy Brake Pads For High-speed Railway Train

Copper-based powder metallurgy brake pad is the key component to ensure the braking safety of high-speed railway train in emergency.Under the condition of high-speed and heavy load,the friction coefficient of copper-based brake pad tends to lose stability and decline.Component control is an effective method to solve this problem.However,the interaction mechanism of various components in copper-based brake pads in the process of high-speed braking and the evolution and failure process of friction film at high speed and high temperature have not been fully revealed,which limits the development and performance improvement of copper-based brake pad.The aim of this paper is to prepare copper-based brake pads that can meet the requirements of high-speed and heavy-load conditions by component control method.Firstly,the continuous emergency braking experiments were simulated to reveal the interaction mechanism of components in the brake pads,and the basic formula with good performance was obtained.The mechanism and regulation process of the components are as follows:The effect of matrix alloying was studied.Prealloyed copper powder(Cu-Fe,Cu-Cr,Cu-Fe-Ti)can improve the friction and wear properties at low speed and low pressure by strengthening the brake pad;Copper nickel alloying can strengthen the copper matrix and friction film,promote the stability of friction surface,thus improving the stability of friction coefficient at high speed and high pressure;The type and content of Fe were studied.It is found that the best type and content of Fe powder depend on its particle size and morphology.Due to the small particle size,the same amount of carbonyl Fe powder produces more interfaces in the brake pad.The plate powder with low strength can not provide enough strength for the matrix.Both atomized iron powder and copper coated iron powder have appropriate particle size and high strength,which strengthen and stabilize the friction surface and promote the stability of friction coefficient in the process of continuous high-speed emergency braking.The adopted iron powder is 22 wt.%atomized iron powder;The effects of Cr and high-carbon CrFe powder as hard metal strengthening components were studied.It is found that both Cr and CrFe can improve the wear resistance and friction coefficient under high speed and high pressure.The improved effect on friction coefficient and wear resistance of Cr powder is better than that of high-carbon CrFe powder,but high-carbon CrFe powder is beneficial to maintain the stability of friction coefficient under different braking conditions.Therefore,Cr powder and high-carbon CrFe are further used together to improve the brake performance.This is due to the formation of porous Cr with low strength during sintering,which acts as the source of fine oxide to strengthen friction film during high speed braking.High-carbon CrFe powder is more stable,which plays the role of bearing load and strengthening friction subsurface layer,and the synergistic effect of Cr and high-carbon CrFe powder improves the stability of friction surface.The adopted content ratio of Cr to high-carbon CrFe is 1:1;The effect of graphite as solid lubrication component was studied.It is found that large-size flake graphite can provide good lubrication,but its strength is low,leading to peel off and increase the wear loss.However,the granular graphite with high strength pinning on the friction surface prevents the crack expansion and the movement of wear debris,thus improves the friction coefficient and wear resistance.Due to the poor lubricity,the high content ratio of granular graphite results in the fade phenomenon of friction coefficient in the process of continuous high-speed braking.The adopted content ratio of flake graphite and granular graphite is 7:6;The effect of MoS2 as solid lubricating component was studied.It was found that during sintering,MoS2 reacts with Cu and Fe in the matrix.In addition to FeS and residual MoS2,the hard phases(Cu2Mo6S8,FeMo,etc.)formed increase the plastic deformation resistance of friction surface and promote the stability of friction surface.The results also show that the particle size of Fe particles decreases and the matrix discontinuity increases,which reduce the deformation resistance of friction surface.In the process of high-energy braking,low deformation resistance and accelerated material movement make the friction surface of the sample with high MoS2 content form a rapidly moving friction film with eddy current structure,which leads to the failure of friction and wear performance.The adopted content of MoS2 is 2 wt.%;The effect of Al2O3 fiber as strengthening component was studied.It is found that Al2O3 fiber can not only improve the friction coefficient at low speed and pressure,but also effectively improve the stability of mean friction coefficient at high speed and pressure.In addition,Al2O3 fiber reduce the wear loss by 45%.This is mainly because that Al2O3 fiber protrudes from the friction surface as the primary plateaus,which hinders the rapid transfer of substance and promotes the formation of secondary plateaus with high strength and stability.The adopted content of Al2O3 fiber is 2 wt.%;Secondly,through the continuous high-speed emergency braking experiment and high-temperature friction experiment,the evolution of friction film and the failure mechanism of friction and wear properties under high-speed and high-temperature conditions were revealed.The obtained basic formula of brake pad was further by optimized by component control.In the process of continuous emergency braking,the friction surface undergoes the following processes:firstly,it is covered by oxide;secondly,a local layered friction film is formed,which is composed of copper rich phase and iron rich phase;thirdly,the internal substance of friction film is refined and mixed evenly;finally,the friction film falls off.When the temperature reaches 600℃ the formed friction film with double-layer structure is easy to transfer on the worn surface of disc.The deformation and softening of copper at high temperature and high stress play a decisive role in the evolution of friction film.The friction film with rapid migration and accumulated failure between the friction interfaces leads to the unstable and fade friction coefficient and the abnormally increased wear loss.Therefore,in addition to the addition of Al2O3 fiber,which can prevent the material migration and strengthen the friction surface,the particle size of Cr and high-carbon CrFe in the brake pad is further increased in order to hinder the migration of friction film and enhance the abrasive wear for removing copper-rich transferred material on the disc surface.This reduces the serious fade and instability of the friction coefficient at high speed and high temperature.As above,a brake pad formula was successfully designed and optimized by using component control method.The results from full-scale dynamometer show that the friction coefficient of the newly developed brake pads can meet the requirements of B.3 in TJ/CL 307-2019 standard within the speed range of 50-380 km/h,and the mean friction coefficient also maintains above 0.35 at 380 km/h.The total wear(0.15 cm3/MJ)is 57%lower than the standard value(0.35 cm3/MJ).In addition,compared with commercial brake pads,the mean friction coefficient of newly developed brake pads is still higher and less affected by pressure changes.The highest temperature on the disc surface is also lower,which indicates that the newly developed brake pads can not only meet the braking requirements of high-speed railway trains with speed grade of 350 km/h,but also have further application prospects in high-speed railway trains with higher speed grade.