Recycling of worn-out vehicular brake pads and discs

Disc brake systems are extensively used in almost all vehicles for braking. The brake pads and discs are usually replaced with new ones before being entirely worn out to maintain their tribological performance. This work highlights the significance of sustainable and environmentally friendly approaches in refurbishing and producing brake discs and pads. The study explores the various aspects of this issue, including the recycling of friction materials recovered from exhaust brake pads and the refurbishing of worn-out brake discs by coating with metallic powders through direct energy deposition. For refurbishing worn-out brake discs, laser deposition technique I indicated to be a promising option as it is feasible to deposit a wide range of coating materials providing a good metallurgical bonding with GCI substrate. Current methods employed to deal with replaced pads and discs primarily involve remelting, with a small proportion disposed of in landfills. Both approaches result in a significant waste of resources and a subsequent increase in CO2 footprint. From a sustainable standpoint, this study aims to evaluate the feasibility of both recycling replaced brake pads and laser deposition of brake discs through a combined environmental and tribological performance approach. The principal focus on coating brake discs is to utilize coating materials that reduce or eliminate critical elements from the composition of the disc (Ni, Co, W, Cu). The recycling of worn-out brake pad materials was investigated by mixing different fractions of the recovered material with a virgin masterbatch to produce low-metallic brake pads. The study shows that this recycling approach is technically feasible, as indicated by tribological tests on the pins that were conducted to select the most promising compositions. The results reveal that the compression strength of the reference pins was maintained up to a 25 wt.% concentration of the recycled material. However, beyond this concentration, there is a decrease in both stiffness and failure resistance. The wear of recycled friction material was found to be very similar to that of the reference sample, suggesting that the recycled material can be effectively used in low-metallic brake pads. On the other hand, the use of powder recovered from the pad underlayer led to an unstable and higher friction coefficient and specific wear coefficient compared to the reference friction material. Regarding brake disc refurbishing, the use of MnS in the stainless-steel coating structure was evaluated as a means of reducing emissions and wear in brake discs. A 5 wt.% concentration of MnS was added as a solid lubricant to the coating formulation, and the microstructural and tribological characteristics of the coated discs (with and without MnS) were compared. The microstructural analysis revealed a homogeneous distribution of MnS phase in the stainless-steel matrix. To evaluate the performance of the coatings, three different friction materials that are commonly used for commercial brake pads were tested using a pin-on-disc setup. The results showed that with the addition of MnS, both the friction coefficient and wear rate were reduced to levels comparable to those of uncoated gray cast iron discs. The tribological behavior of the MnS-containing coating was found to be the best when sliding against a Cu full friction material. Another type of coating investigated is constituted by a medium carbon steel powder (Ferro 55), a commercially available Ni-free material. The tribological properties of this coating were investigated using a pin-on-disc test with two different friction materials: Cu-free and ECOPADS/B. The Ferro 55 coating exhibited a relatively high hardness of 700HV, which is higher than that of gray cast iron (GCI). However, the results revealed that both types of friction materials had very similar wear behavior, friction coefficients, specific wear rates, and total emissions when sliding against the coated disc. Although these tribological data were slightly higher than those obtained with an uncoated disc, this finding suggests that further improvement of the top coating composition is necessary to achieve better performance. As a third route, the potential of Fe3Al coating material as an environmentally friendly alternative to coatings containing critical elements for brake discs was investigated. A Ferro 55 buffer layer was applied to enhance coating quality and prevent hot crack formation during solidification. Microstructural analysis of the coating cross-section revealed diffusion of the buffer layer into the Fe3Al coating, forming a combination of Fe3Al, Fe, and Fe3AlC0.5 phases. Tribological properties of the Fe3Al coating were evaluated through pin-on-disc tests using two different copper-free friction materials cut from commercial brake pads. Results indicated a comparable coefficient of friction with an uncoated disc, with a slight reduction in particulate matter emissions. However, further development and the use of reinforcement phases are necessary to enhance the wear resistance of this coating. Overall, this part of the research highlights the potential of Fe3Al coating materials to mitigate toxic emissions from brake discs while maintaining comparable wear resistance. Such eco-friendly coatings have the potential to address environmental concerns and promote sustainable development.