Mechanism Of High Temperature On The Friction Interfaces For Friction Pairs Of High-speed Trains Induced Braking Instability

The brake system is one of the key components to ensure the safety and comfort of high-speed trains.At present,copper metal matrix composites(Cu-MMCs)paired with cast steel were generally used in high-speed trains to achieve deceleration or park.However,at high speed and high energy,high temperature generated by the friction interface,caused a serious decline in the friction coefficient of Cu-MMCs paired with cast steel,even leading to friction instability.In order to ensure the reliability of high-speed train braking system,it is necessary to investigate the tribological behavior and mechanism of Cu-MMCs paired with cast steel at high temperatures.In this paper,Cu-MMC had been designed and prepared,which met the requirements of high-speed.Friction experiments on Cu-MMC paired with cast steel were conducted at controlled ambient temperatures,to investigate the effect of high temperature on the wear mechanism and material evolution.The main research contents and as follows:(1)The friction properties of Cu-MMC at ambient temperatures ranging from 25 °C to800 °C was studyed.As the ambient temperature increased from 25 °C to 400 °C,average friction coefficient and mass loss increased.As the ambient temperature increased to 600 °C,average friction coefficient slightly decreased and wear loss increased.When the ambient temperature reached 800 °C,average friction coefficient and mass loss decreased sharply.With the increase of ambient temperature,the wear mechanism of Cu-MMC changed from slight abrasive wear at 25 °C to severe oxidation,abrasive wear at 400 °C and 600 °C,and the worn surface showed delamination wear and oxidation at 800 °C.(2)Wear mechanism and material evolution on the surface of Cu-MMC at 600 °C and800 °C were investigated.At 600 °C,the wear mechanism of Cu-MMC changed little.The lowest wear rate was exhibited in the initial stage of friction,and the friction coefficient showed an increasing trend.With the increase of friction time,wear rate increased and the friction coefficient changed to a decreasing trend.During the smooth stage,showing the highest wear rate,and there was nearly no friction film formed on the worn surface.At800 °C,the highest wear rate was exhibited in the initial stage of friction,and the friction coefficient showed a rapid decreasing trend.As friction time increased,wear rate gradually decreased,friction coefficient decreased at a slower rate,and gradually reached a smooth stage.The friction film on the surfaces of Cu-MMC gradually became uniform and continuous.(3)Co-based coatings were prepared on the surfaces of cast steel brake discs by Laser cladding,and the tribological behavior and wear mechanisms of different brake pairs were investigated at 25 °C-800 °C.At 25 °C and 600 °C,different brake pairs exhibited similar tribological properties,and Co-based coatings was damaged less than cast steel brake discs.At 800 °C,the wear mechanism of different brake pairs showed significant differences,and the worn surfaces of Cu-MMC sliding against cast steel showed the characteristics of peeling and oxidation,and the worn surface of Cu-MMC was almost covered by a uniform and continuous friction film.The wear mechanism of Cu-MMC sliding against Co06 coating was severe abrasive wear,peeling and oxidation,and friction film on the worn surfaces of Cu-MMC was uneven and poorly continuous.Co-based coatings were slightly damaged at all ambient temperatures.