Let’s first understand how the performance indicators of the material at high temperature will change: the mechanical properties of the material are affected by the temperature, with the temperature increases, the strength, stiffness, hardness decreases, plasticity increases.
At the same time, at higher temperatures, the duration of the load also has an impact on the mechanical properties, which will produce significant creep.
Frequent braking will cause the brake pad to heat up due to friction, which will cause the brake pad to lose strength, stiffness and hardness and increase plasticity.
At higher temperatures, when we apply the brakes, the brake pads are under constant stress, and even if the stress is less than the yield point at that temperature, the material will slowly deform plastically.
Once creep occurs, the material will eventually fracture in severe cases.
By the way, I would like to explain the mechanical properties of metal materials at room temperature, so as to make a comparison with the mechanical properties at high temperature.
At room temperature, the material will deform plastically only when the stress exceeds the yield point.
For example, Q235, as a common grade of carbon structural steel, its mechanical properties are as follows: yield strength: the lowest stress value when the material begins to produce significant plastic deformation.
Tensile Strength: The maximum stress value that the material is subjected to before breaking.
At room temperature, if the stress value is less than the yield strength, then the material deformation will disappear after the force is removed.
Only when the stress value is greater than the yield strength, the plastic deformation of the material can occur, and the deformation will not disappear after the force is removed.
In high temperature, Q235 steel will be under the effect of constant temperature and stress for a long time, even if the stress is less than the yield point of the temperature, it will slowly produce plastic deformation, and plastic deformation is permanent deformation, for precision mechanical parts, even a small amount of plastic deformation will lead to a decline in the accuracy of the parts and the formation of failure.
I personally believe that the root cause of the decrease in friction coefficient is the decrease in performance of brake pads at high temperatures, such as creep, and the decrease in strength, stiffness and hardness, which means that the brake pads become softer and, therefore, affect the brakes.
The brake pads of the car:
 Ultrasonic nondestructive testing of basic knowledge of metal materials: common carbon structural steel and low-alloy high-strength structural steel
 “Engineering Materials and Forming Technology” Chapter 2 material properties: the high-temperature properties of materials