Relationship between Mechanical Behavior and Microstructure Evolution of Sintered Metallic Brake Pad under the Effect of Thermomechanical Stresses

Sintered metallic composite is widely used as brake pad material for high energy railway thanks for its good resistance to severe solicitations caused by braking loads. Despite its efficiency, the degradation of the material properties under the effect of brake loads has been noticed in literature which is undoubtedly induced by the microstructure evolution. However, the microstructure evolution and its relation with mechanical behavior have so far not been intensively investigated due to the complexity of braking solicitations. To solve the problem without tackling it in all its complexity, two experimental tests were proposed where physics are decoupled; but still inspired by the braking sequence in terms of applied temperature and compressive load. The first one is the thermal solicitation test where a temperature gradient from 400°C to 540°C was applied to the material. The second one is the thermomechanical test where a compressive load at 20 MPa was applied under the same thermal gradient. The experiment time is fixed for two minutes, equivalent to the time of one braking stroke. Besides, the local microstructure evolution of the sintered metallic brake pad was characterized by Electron Microscopy (SEM) coupling with Energy-dispersive X-ray Spectroscopy (EDS) and X-ray microtomography. The evolution of mechanical properties was characterized by a series of compressive tests equipped with a Digital Image Correlation (DIC) for analyzing deformation behavior. Based on the deformation behavior characteristics, the considered thermal and mechanical solicitations have no separate effect on the mechanical properties of the material. The sole evolution of mechanical behavior is due to the coupled thermomechanical solicitation, which increases the hardness of friction material. From the strain field analysis, the evolution takes place on the strain lines determined by the compressive test, which strongly depends on the distribution of graphite inclusions in the microstructure. The change in mechanical behavior is induced by the local microstructure evolution. Indeed, thermomechanical stresses cause the densification of the graphite in the normal direction, this structural change induce some shear cracks in the basal plane. In terms of the metallic matrix, the segregation of carbon in steel is investigated as a reason for the increased stiffness.