Influence of Graphite Morphology on the Frequency Response Function of Brake Discs

The automotive industry has evolved hugely in recent years. Components such as brake discs not only ensure the customer safety, but also provide comfort and a sense of well-being. This new concept of quality is defined by an absence of noise which is related to the natural frequencies of the brake discs. This issue is of significant concern in future years. As for the mechanical properties, these natural frequencies are directly influenced by the amount, shape and type of the graphite.

To further understand the influence of graphite morphology on the frequency response function (FRF) of brake discs, six gray cast irons with carbon equivalents (CE) between 4.30 and 4.47 were produced. A total of 25 brake discs were cast from each melt. An inoculant rich in Ba was also used. The FRF was measured by an impact test. Graphite flake shape parameters were determined by a commercial image analysis software. Hardness (HB) and tensile strength were also evaluated.

As expected, the type of lamellar graphite changes from A to C when CE increases, decreasing significantly the FRF. This type C is characterized by a lamellae thickening, but also by the precipitation of graphite with starfish shape (kish graphite) and by the formation of big clusters of graphite. A clear inverse relation between the number of primary kish graphite particles and natural frequencies was found. The drop in FRF is also related to a loss of mechanical properties.