Influence of the chemical composition on the generation of cracks in brake discs

Thermoelastic instabilities in the contact of brake friction material cause hotbands and hotspots on the surface of brake disc. These phenomena generate thermal stresses that result in generation of cracks, which limit the lifetime of the discs. In the present work, the influence of the chemical composition of brake discs on the thermoelastic behavior of the system and on the lifetime of the discs was investigated. The experimental evaluation was carried out in an inertial dynamometer using the SAE J3080 standard procedure applied on a brake system. Two discs (namely A and B) with different chemical compositions were subjected to the tests. The brake pad composition was kept fixed. The thermoelastic effects on the inner surface of the disc were observed by contact (thermocouple) and noncontact measurement techniques (thermography), as well as through photographic images of the disc’s surfaces. Disc A showed negligible amount of Nb while disc B exhibited 0.360%. Besides, disc B presented approximately 15% more content of C (Carbon) than disc A. Disc B showed twice the lifespan of disc A. This can be attributed to the smaller thermal gradients observed in disc B, which is in turn related to the higher thermal diffusivity (htd) measured for this rotor. The htd is explained by a greater density of graphite (related to a greater amount of C in the composition), as well as a finer (influenced by Nb) and well-distributed graphite flakes seen in case of disc B.