Evolution Analysis of Wear Behavior of Brake Pad Friction Block under Multiple Braking Conditions

The air brake system is one of the crucial safeguards for the safe operation of railway vehicles. It achieves braking by converting the train's kinetic energy into thermal energy through the friction contact between the brake disc and the brake pad friction block. During the train's service life, the friction block is used for the frequent stopping, speed reduction, and speed adjustment, and these actions can lead to continuous wear of the brake pad friction block until it reaches its wear limit. The wear of the friction block not only affects the tribological behavior at the brake interface but can also, in severe cases, threaten the safety of the train's braking. Therefore, studying the wear behavior of the friction block during its service is of significant importance. Different service environments, braking parameters, material compositions, and friction block structures make the wear behavior of the friction block extremely complex. Scholars have primarily studied it through experiments and simulations. However, existing research is generally limited to analyzing the damage to the friction block, the distribution of contact pressure and the evolution of wear under single braking conditions. Yet, in the actual service process, the friction block has to undergo multiple braking conditions repeatedly. Current research seldom analyzes the impact of multiple braking conditions on the wear behavior of the friction block. To clarify the wear behavior of friction block under multiple braking conditions, single and multiple braking tests were conducted using a scaled braking test bench. Furthermore, a thermo-mechanical coupled wear analysis was performed using finite element models to explore the evolution of contact pressure and wear during the braking process. The results indicated that after multiple braking conditions the size of spalls on the friction block surface increases, the number of spalls decreased, and edge spalling was more pronounced. The number and size of matrix cracks near the friction interface had increased; the average crack length was 51 μm after a single braking and increased to 149.2 μm after multiple braking conditions. During the running-in process, severe wear was observed at the cutting-in end of the friction block, and significant wear in the middle of the friction block was noted during the first braking, with a maximum wear depth of 56.1 μm and a minimum wear depth of 37.8 μm, showing a difference of 18.3 μm. In subsequent braking processes, the wear of the friction block became more uniform, with the difference between the maximum and minimum wear depths within 1 μm, and the overall wear depth continually increased with the number of brakings. The distribution of contact pressure of the friction block and the evolution of the main wear areas were influenced by the initial wear state, thermal expansion behavior and wear behavior. The research results revealed the evolution law of the wear behavior of the friction block under multiple braking conditions, which could provide necessary theoretical support for the design optimization of the friction block.