Towards Sustainable Braking: Effects of Alternative Ingredients on the Tribological Properties of Friction Materials

The most important safety device for commercial vehicles is the brake system. It has to satisfy multiple requirements such as braking performance, wear resistance and low noise production. In the last years, a further requirement has been gaining increasing attention: their particulate matter emission. Indeed, brake systems are one of the most relevant sources of non-exhaust traffic-related emission PM. These emissions have been shown to negatively affect both the environment and the human health, urging authorities to take legislative action. This work is focused on improving the sustainability aspects of brake friction materials. This was done by following two different approaches, both of which involved the use of alternative ingredients in the friction material composition. The new ingredients were chosen based on their properties while also considering aspects related to an eventual industrial application. The first approach involved the use of a natural ingredient, rice husk, as an ingredient in the composition of friction materials; whereas the second approach dealt with the substitution of the conventional phenolic resin binder with benzoxazine resins. The tribological and PM emission properties of friction material samples produced using these ingredients were investigated via tribological testing, and were correlated to the results of the characterization analyses that were performed on the worn surfaces, on the collected emitted particles, and on the newly adopted ingredients themselves. Rice husk is a widely available byproduct of the agricultural industry, and possesses some peculiar properties related to its considerable inorganic content that separates it from the other lignocellulosic materials. Friction materials modified with rice husk were tested with a Pin-on-Disc tribometer and with a reduced-scale dynamometric bench while also monitoring their PM emission. Overall, the adoption of rice husk showed promising results, preserving the friction material coefficient of friction, PM emission and wear resistance. However, when higher temperatures were reached, a loss of performance and durability was observed. Nevertheless, this temperature range was above that associated to common LDV brake system operation, which this application is targeted to. Benzoxazine resins are a relatively new class of binders that display some attractive properties when compared to the conventionally used phenolic resins, such as more forgiving storage conditions, improved thermal stability and customizability of their properties. Two of the simplest representatives of the benzoxazine resin family were hereby adopted. An initial characterization of the thermal behaviour of the resins was first carried out to design the production process of the friction materials containing them. Friction materials bound by benzoxazine resins and by a commercial phenolic resin were tested by Pin-on-Disc and reduced-scale dynamometric bench testing while monitoring their PM emission production. The substitution of phenolic resin with benzoxazine resin as friction material binder proved to be successful, leading to lower wear of the samples, which would translate to a lower environmental impact.