Analysis of the Influence of Environmental Humidity on Stick–Slip Vibration of Lightweight Brake Friction Pairs in Urban Rail Trains

The experimental results demonstrate that environmental humidity has a significant influence on the frictional and dynamic responses of the lightweight braking friction pair. Under identical braking conditions, an increase in environmental humidity leads to a noticeable rise in the static friction coefficient from 0.39 to 0.51. At the same time, the stick–slip vibration period is prolonged by approximately 35%, accompanied by a substantial amplification of vibration amplitude and increasingly pronounced nonlinear dynamic characteristics.

The phase-plane trajectories also exhibit larger oscillation loops and more irregular evolution patterns under high-humidity conditions, indicating stronger dynamic instability within the friction system. Mechanistic analysis reveals that humidity-induced changes in the tribological interface play a crucial role in regulating the vibration behavior. Under low-humidity conditions, a continuous and compact carbon-based friction film can be readily formed on the friction surface.

This stable tribofilm effectively reduces direct asperity contact, promotes uniform stress distribution, and facilitates smooth sliding motion, thereby suppressing the occurrence of severe stick–slip vibration. In contrast, under high-humidity conditions, the presence of water vapor accelerates tribofilm degradation and promotes interfacial oxidation reactions. As a result, the friction layer becomes discontinuous and locally damaged, while contact plateaus exhibit greater heterogeneity and structural looseness.

These microstructural degradations lead to uneven stress distribution and intermittent energy release at the interface, which significantly intensifies stick–slip vibration and dynamic instability of the braking system. Overall, this study clarifies the influence mechanism of environmental humidity on the stick–slip vibration behavior of lightweight braking friction pairs in urban rail systems. The findings provide important theoretical insights into the tribological regulation of friction-induced vibration and offer a useful reference for the optimization of friction materials and the vibration suppression design of urban rail braking systems operating in high-humidity environments.