Whatever humanity does today, it does it on a very large scale. And the consequences are large as well! In the automotive area, I was clearly aware of CO2 emissions and global warming issues. A few grammes of CO2 per kilometer leads a whole planet (ours!) to become much less hospitable than it currently is… particularly when the whole humanity drives each year about a light year!
But at this scale, other challenges arise such as brake pad wear particles emission. Every time one uses the brake pedal, a little amount of wear occurs on the pads. The paper from N. Kalel et al.  I would like to present here raised my awareness on this, but also thoroughly assesses solutions by comparing different pad compositions (see Figure 1). Currently, copper is one of the key components of the brake pads even if it represents a low percentage of the different components volume (see Figure 2). However, the copper dust that ends up in the rivers and lakes is clearly unhealthy for them. A few governments in the world have already ruled the use of copper in brake pads, and brake pad makers propose solutions. It is true that regenerative brakes lead to reduced use of the brake pads while other improvements come from the discs. But it is not enough and the authors explore solutions.
So, what are the alternatives? Before diving into the conclusions of the N. Kalel et al. , one needs to go through the expectations weighting on the candidate to replace copper! To start with the basics (see Figure 3), the brake pads are attached to hydraulic pistons that press them heavily against a disc. This disc rotates together with the wheel. When the pads rub against the disc it slows the rotation of the wheel and eventually stops the wheel… and hopefully the vehicle to which the wheel belongs!
As brake pads are a key security component for any vehicle, they need to behave in a very consistent way despite the very different environment conditions they endure. The kinetic energy dissipated from the vehicle speed reduction is directly converted into heat and evacuated through the pads and the disc. The energy to dissipate can be “ludicrous” depending on the vehicle which means the pads need to produce a similar friction at ambient temperature and potentially a few hundreds °C. Besides, the pads should last a least a few tens of thousands kilometers and therefore have a moderate wear rate. The pads should also minimise the noise produced while braking. The replacement material should also be sufficiently cheap and abundant. At last, the successor of copper based pads should be more environment friendly even if they are used on everybody’s car.
The replacement material N. Kalel et al.  have explored is stainless steel. As it comes in various grades, and that their mechanical and thermal properties can be different, different candidates have been explored such as 304, 316, 410 and 434. The performance summarised by friction coefficient value and consistency and wear rate can be broken down into different material properties such as hardness, density, shear strength, thermal conductivity or surface energy. They all have a meaning and an influence on the braking quality. After carefully measuring the properties of the stainless steels and the copper, they also tested the braking quality itself within different braking cycles. The tests allowed to measure the friction coefficient under different conditions and the mass lost through wear. Even if the metal particles represent only 3% of the pad mass, they have a key role.
It appears that steel particles can completely replace copper particles in the brake pad composition. Because the criteria to compare the pads are numerous, a MOORA method has been implemented. On top of the ranking there is the 316 grade. The latter even beats the copper based pads in this study. However, another study from the same authors pertained to noise and vibration performances… and the 316 grade came last in this when on this other criteria series. The rightful candidate may not be Stainless Steel 316 but an alternative explored here such as the 410 or 304 grades.
To know more about N. Kalel et al. study and methodology, feel free to have a look at their very interesting paper .
 N. Kalel, B. Bhatt, A. Darpe, J. Bijwe, Copper-free brake-pads: A break-through by selection of the right kind of stainless steel particles, Wear (2020), doi: https://doi.org/10.1016/j.wear.2020.203537