The fracture surface
colour at the edge of the rim indicated a high temperature reached during the brake application. On the other
hand, the slight variations in hardness observed between the hub and the friction surface together with the
lack of spherical pearlite and carbides at the grain boundaries suggested the maximum temperature was less
than 500 C, as confirmed by FEA. Traditionally, brake discs are manufactured in gray iron with a predominantly
pearlitic matrix due to its chemical-physical properties, mechanical strength and wear resistance [10].
However, graphite morphology, matrix microstructure, surface degradation as well as alloying elements
greatly influence the fatigue performance of this kind of material. In using gray iron, is has been observed
mechanical and thermal properties as well as fatigue strength may be actually improved through the addition
of alloying elements, such as Si in high content (more than 2.5%) and Mo, Ni and Ce in traces in order to
have a graphite refining effect, respectively [6,10,11]. The gray iron investigated in this paper didn’t meet
these characteristics. In particular, the Si shows a content equal to 1.56% and the traces elements above mentioned
are missing.