The service life of sintered friction materials developed for train braking applications is slowly increasing, as they are less and less used for the sake of electric braking technologies. Recent feedbacks from field observations showed that a decrease of the friction coefficient can be observed after an extended use. Iron-based materials are particularly concerned, as they are superficially and deeply impacted after an exceptionally long exposure to standard service loads. This has led to the hypothesis of deep material changes caused by a long environment exposure, modifying their tribological behaviour. In order to verify this assumption and to check the determining factors of the friction loss, an experimental protocol has been developed to compare properties and behaviour of artificially weathered materials to those of their as-manufactured and post-service states.
The developed strategy consists in confining as-manufactured materials into a climatic chamber, exposed for one week to a wet air atmosphere (95% of relative humidity). During the ageing process, some specimens were submitted to braking sequences. After weathering, materials have been submitted to friction performance tests, and their surface and in-depth microstructure have been characterized by scanning electron microscopy and EDX analysis.
Weathered materials exhibit trends similar to those aged in service, though less pronounced, in terms of friction performance loss and surface and microstructure changes. Overall, friction is significantly reduced after ageing, in accordance with the development of oxides in their porosities, and with the changes in the tribological mechanisms. These trends appear with the sole exposure to humidity and are exacerbated by the addition of friction during weathering.
Results show that the sole wet air exposure is not enough to alter friction and the superposition of friction is a key factor of the performance loss. Developing a simplified weathering test requires to introduce the coupling between tribological, mechanical, thermal and chemical loadings.
A simplified weathering test has been developed, accelerating the ageing process of friction materials to obtain the performance loss within a week. Combined with complementary characterization methods, it brought a better understanding of the friction loss observed after an exceptionally long use in service. In the future, a deeper investigation of chemical changes in friction materials and their impact on the tribological circuit evolution will help understanding the source mechanism of friction decrease. This will lead to the development of an industrial weathering test, to quickly evaluate friction material resistance to ageing and technical solutions to improve it.