| The high-speed railway is one of the China’s advanced technologies.As a necessary guarantee for the safe operation of high-speed trains,the brake pad and disc are required to stop the train in emergency situations.As the key material,the brake pad with high and stable friction coefficient,low wear rate,and appropriate strength is desired for the stable brake.However,the brake pad is composed of various components with different properties and different dimensions.What is more,the friction and wear mechanisms of each component are not the same,which brings great difficulties to the development of materials.With the development and maturity of technology,the operating speed of high-speed train will be further improved,and the brake pad material are facing more severe challenges.Only by understanding the law of the friction and wear of materials under different working conditions and making connection of the wear mechanisms with each component,the composition design and preparation process can be further improved,and the safety of the train can be guaranteed.The study is based on the practical working conditions(30～380 km/h)of high-speed train and conducted a series of inertial braking friction experiments.Different friction and wear mechanisms with different working conditions were revealed by designing friction materials with different metal matrix,different abrasive particles and friction film.Theoretical guidance and data support for material design and service application was provided as follows.(1)Copper powder and iron powder were used as the main matrix components,and the samples with different copper/iron ratios(Cu/Fe(wt.%)=7.5～0.26)were prepared.The intrinsic relationship between matrix composition and friction behavior was revealed by studying the microstructure,composition and peel strength of the friction film.The results showed that the matrix composition affected the friction and wear of the material by affecting the formation and destruction of the friction film:the lower the ratio of Cu/Fe,the easier to form a continuous and low-strength friction film,and the lower coefficient of friction and wear rate of the material.When the Cu/Fe ratio was increased,the formation of the friction film becomes difficult,the peeling strength was lower,and the material further showed a high coefficient of friction and a high wear rate.In addition,the peeling strength of the friction film affected the stability of the friction coefficient of the material at the high-speed braking:the higher the peeling strength,the more stable the friction coefficient of the high-speed repeated braking,and the lower the wear rate.When Cu/Fe is 2.4,the material has a high and relatively stable coefficient of friction with a moderate wear rate.(2)The friction materials were prepared with different types of abrasive particles(TiC,Al2O3,SiO2,TiO2)and content(0～12vol%).The friction behavior and failure mechanism of the pad with different abrasive particles were studied.The results showed that the hardness of the ceramic component,the interface with the matrix and the thermal expansion match determine friction-increasing effect,and the friction-increasing effect was also affected by the braking speed.When the braking speed did not reach the condition of the delamination,the friction-increasing effect of the abrasive particles depended on their own hardness and contents.The more the content and the harder the particles,the more significant the friction-increasing effect.After the delamination,the friction-increasing effect of the abrasive particles mainly depended on their interface with the matrix:the better the bonding,the more significant the friction-increasing effect.With high hardness and good inter-diffusion with the iron matrix,TiC can meet the low-speed and high-speed braking performance requirements,and it exhibited the best friction and wear performance when added at 6 vol%.(3)Based on the research results above,the brake pad friction material was prepared with the appropriate amount of graphite powder and other strengthening constants.The brake tests with initial braking velocity of 50～380 km/h were conducted to evaluate the friction performance of the brake pads.The results showed that in the range of 50～380 km/h,the average friction coefficient of the material showed a three-stage change of "decrease-increase-decrease":at 50-160 km/h,the coefficient of friction decreased from 0.416 to 0.382;at 200-320 km/h,the coefficient of friction gradually increased and reached a maximum value of 0.416 at 320 km/h;at 350-380 km/h,the coefficient of friction decreased and finally reduced to 0.386.The average coefficient of friction was varied by 8.9%over the entire speed range,lower than the limit of 15%required by industry standards.At different braking speeds,the average coefficient of friction was highly related to the evolution of the friction film.The wear rate of the material changes with speed as a"V" shape,which can be divided into:slight wear,moderate wear and severe wear.Slight wear occurred in the 120～200 km/h,the main wear mechanism was abrasive wear,plough wear and oxidative wear;medium wear occurred in the 50～80 km/h and 250～320 km/h stages,and the wear mechanism was adhered and delamination;severe wear occurs at the speed over 320 km/h,and mainly caused by severe delamination.The lowest wear rate was 0.028cm3/MJ at 160 km/h;the maximum was 0.236 cm3/MJ at 380 km/h,meeting the standard requirements of 0.35 cm3/MJ.(4)In order to reveal the mechanism of the three-stage friction curve of the brake pad and study the change of friction mechanism under different braking speeds,the pad with prefabricated tribo-film(PTF)was prepared.Interestingly,the PTF sample showed a similar friction behavior to the original brake pad:the three-stage coefficient of friction curve and the "V" shape of the wear rate curve,indicating that the friction film determines the brake friction behavior of the material.Since the properties of the friction film were relatively stable,as long as it was not destroyed,the material will maintain a relatively smooth coefficient of friction and low wear rate.At this time,the friction force was derived from the meshing and ploughing action of the friction film and the disc;when the friction film underwent high temperature and high shear stress,severe delamination occurred,the braking performance was significantly degraded.The friction behavior was related to the delamination and shearing of the friction surface.(5)Based on the braking behavior of the brake pad prepared above,112 sets of brake friction tests including three conditions of initial braking speed,braking positive and braking inertia were designed,covering all possible braking of high-speed trains.In the study,the maximum temperature of the pad surface Tmax was used as the intermediate variable to construct the frictional instability map and wear mechanism map under multi-case conditions.The boundary conditions of the friction stable/unsteady zone,severe wear zone and the best friction performance zone of the material was determined,which provides a reference for train stable braking. |
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