Abstract
Premature failure by cracking is one of the major problems for truck brake disc lifespan. One improvement way is to evolve disc material with the aim of increasing their braking resistance. We chosed to modify the graphite morphology and to study a nitrogenenriched cast iron with short graphite flakes and a cast iron with a microstructural gradient composed of small graphite flakes near the surface and coarse graphite flakes in the depth. These evolved materials were compared to a reference grey cast iron commercially used for truck brake discs. The first step consisted in braking test on a lab-scale tribometer to check their friction behaviour and to compare their performance to the reference material. The test consisted in a series of stop-braking and a series of slowdowns with heat accumulation. The results showed that the reduction of the graphite flake size does not have a significant impact on friction behaviour. The second step consisted in thermal cycle testing to evaluate the thermomechanical resistance of the new materials under cyclic loading. A specific test was developed, taking into account the thermal stresses induced by braking. It is shown that the cracks propagation is limited with short graphite flakes. To develop this test to be as close as possible to real brake conditions, we based the test parameters choice on a previous coupled numerical-experimental study on full-scale braking.
KEYWORDS - brake disc, evolved cast irons, braking, thermal fatigue, cracks