The Role of Aluminum on the Wear Behavior of Grey Cast Iron Brake Discs

Brake emissions, mainly fine and ultrafine particles of metallic dust and other materials, are as or even more harmful than exhaust gases, causing serious health problems in both adults and children. The most worrying particles are PM10, those with a diameter of less than 10 microns, since they have the ability to infiltrate lung tissue causing significant damage. This growing and demanding problem will be regulated by the Euro 7 standard, which will drastically reduce polluting emissions from passenger cars from 2027 and from heavy-duty vehicles (HDVs) in 2029. Elements such as asbestos, a material that was replaced due to its high toxicity, or copper, a key component in current brake pad coatings, must be avoided to reduce brake dust pollution. Modifications to brake discs through metallurgical solutions, heat treatments, or special coatings, making them more resistant to wear, also contribute significantly to reducing emissions into the atmosphere.

The goal of this research was to improve the conventional grade Fe-C-Si gray cast iron with specific contents of aluminum, either by replacing silicon (3.27%Al and 0.21%Si) or by adding it as an alloying element (4.37%Al and 1.81%Si), analyzing its effect on the microstructure, mechanical properties, wear resistance and particle emissions. Several brake discs (without Al, with 3.27%Al and with 4.37%Al) inoculated with 0.15 % of a ZrMn inoculant were melted for this purpose. Metallographic analysis was conducted. Main phases formed were identified using SEM. Wear resistance was evaluated as mass loss by means of pin-on-disc (PoD) testing. The concentration of airborne wear particles (PM10) was measured continuously during testing with a suction pump SKC. Worn surfaces of pin and discs were also examined.

 
Type A graphite was predominant in samples without aluminum, leading to a refinement of graphite with the formation of some type E because of the presence of Al. Microstructure was fully pearlitic for samples without aluminum and with 3.27%Al, but it is mainly ferritic for samples with highest content (4.37%Al). No amounts of ledeburite, cementite and carbides were detected. Polygonal Mn-sulfides were noticed in all discs. Fe-Si-Al intermetallic compounds were observed in samples with 4.37%Al. Highest values of mechanical properties were obtained for sample without silicon (265 MPa and 211HBW) because of the changes in the crystal lattice when silicon is replaced by aluminum and by the effect of aluminum on the solubility of carbon in the liquid and the austenite. Superior wear behavior was discovered for samples alloyed with aluminum, improving a 60% the wear resistance and a 30% PM10. Oxidative and adhesive wear was found to be prevalent in all samples.