| Automobile brake which convert the kinetic energy into heat energy to realize the brake by the friction between the brake friction material and metal dual parts,is the most unstable links in the braking system.Inactive of braking materials in high temperature is the main reason to cause the brake failure.Impact of brake parameters such as velocity, pressure and time to the material can be eventually attributed to the influence of surface temperature,so it is necessary to study the rise of temperature of the brake material surface and the change of friction mechanism under different braking parameters.In this work,two sorts of representative hybrid fiber reinforced resin matrix brake friction materials and one kind of commercial brake pad material were selected for the study.Friction and wear properties of friction pairs under different temperature and the influences of friction time on heat fading were tested on Chase friction tester, MM 1000- Ⅱ friction tester was also used to simulate the reality vehicle braking conditions so as to obtain the surface temperature and the change of tribological properties of the friction material.The change rules of tribological properties of brake materials under different working conditions were obtained,the key factors to impact the tribological properties of brake materials were analyzed,the friction and wear mechanisms of the new automotive brake friction material in the braking process were clarified.The experimental results show that:friction coefficient of brake materials increased slightly in first then decreased with the increasing of brake disc temperature,the friction coefficient increased from 0.29 at10℃ to 0.32 at150℃ then down to 0.21 at 350℃;The impact on the surface temperature of the brake pressure was less,the temperature of surface increased from 135℃ to 195℃ since the the brake pressure increased from 0.5MPa to 3MPa,Friction coefficient and decreased with the increasing of brake pressure;The surface temperature increased with the increase of the brake initial velocity,friction temperature could reach 340℃ at 4500r.min-1,The friction coefficient first increased and then decreased with the increase of initial velocity,friction was not stable under the high speed, and the amount of wear were large.After prolonged friction,the temperature of friction interface reaches a certain stable value, the surface temperature could reach 410℃ (under 690r.min-1,624N),and friction coefficient fluctuated in the initial stage and then stabilized.Effect of temperature on the properties of the material is mainly manifested in three aspects:first,thermal decomposition,the material with more resin has greater temperatures sensitivity,due to the melting of the resin at high temperature,the friction state changed from the dry friction to boundary friction which lead to the reduce of friction coefficient. Second,the thermally oxidation,because the metal material and organic compounds was easily oxidized at a high temperature;and the hardness and strength of oxides are low,the bearing capacity oxides in the friction process was low and can be easily cut into debris.Third,thermal fatigue,cyclic thermal stress and repeated shearing would lead to the fatigue damage of materials in contact surface.Within a certain range of temperatures and pressures,the increase of temperatures and pressures contributed to the formation of the surface film,stabled the friction. But the high pressure and temperature, could lead to serious material surface asperity deformation and friction membrane damage which caused the friction instable.The influence of temperature on the properties of the brake materials has relationship with material composition to a extent,the material with more resin has greater temperatures sensitivity.The relationship between braking parameters and friction performance is mainly affected by the brake pad materials and the rise of temperature. |
