Addition of Alloying Elements as a Potential Tool for Improving the Wear Behavior of Grey Cast Iron

The release of fine particles (PM2.5 and PM10) generated by wear of brake pads and discs during braking represents a significant public health issue. These non-exhaust emissions are often more toxic than those from internal combustion engines and account for 55% of transport-related pollution, leading to major cardiovascular and respiratory problems.

Historically, brake emissions have been largely overlooked until the European Union introduced the upcoming Euro 7 regulation, which will drastically reduce pollutant emissions from passenger vehicles from 2027 onwards and from heavy-duty vehicles (trucks and buses) from 2029 onwards.

Many technologies are currently being developed to mitigate this issue, including heat treatments, application of hard coatings on brake discs using thermal spraying techniques (HVOF) or laser cladding, development of new brake pad materials, and brake dust filtration systems to capture particles before they are released into the atmosphere. Microstructural modification through metallurgical solutions, involving the controlled addition of alloying elements, also plays a key role in reducing particulate emissions.

The aim of this research is to analyse the effect of adding selected carbide-forming elements (Nb, Cr, Mo, Ti) on wear resistance and particle emissions, due to their ability to modify the crystal structure and form very hard carbides. To facilitate processing, rectangular plates of 150×75×20 mm were used instead of real brake discs with much more complex geometries.

Type A graphite was the predominant form regardless of the additions. In all cases, the microstructure was fully pearlitic, with no ledeburite or cementite observed. Complex carbides of types MC and M7C3 were detected depending on the alloying element added. The best mechanical properties were obtained with chromium addition. Improved wear behaviour was observed in Mo-alloyed samples, with a 60% increase in wear resistance and a 30% reduction in PM10 emissions. Oxidative wear was the dominant mechanism. Fe, Zn, Al and Ba oxides appear to be the main emitted particles.