The increasing interest in the emission from the disc brake system poses new challenges for the characterization approaches used to investigate the particles emitted from the wearing out of the relevant tribological systems. This interest stems from different factors. In the first place, a thorough characterization of brake wear particles is important for a complete understanding of the active tribological mechanisms, under different testing and servicing conditions. This information is an important prerequisite not only for the general improvement of brake systems, but also to guide the development of new materials for discs and brake pads, responding better to the specific requirements, including not only performance, but also the emission behavior. In this review paper, the main material characterization protocols used for the analyses of the brake wear products, with particular regard for the airborne fraction, are presented. Reliable results require investigating the fine and ultrafine particles as concerns their composition together with their structural and microstructural aspects. For this reason, in general, multi-analytical protocols are very much recommended.

A significant fraction of the non-exhaust particulate matter emissions from vehicular traffic comprises fine particles from the wear debris of brake pads and discs. Recent studies have shown that these emissions can be consistently reduced by using wear resistant disc coatings. This study thoroughly analyses the debris produced by a low-met brake pad, which is dyno-bench tested against both cast iron and WC-CoCr-coated brake discs. To achieve this, particles in the size range of 2.5 µm to 30 nm were collected and characterized. The results showed a consistent reduction in the particle emission as well as in the concentration of iron oxides, which are mainly released from the disc tribo-oxidation in the coated disc. Furthermore, a few tungsten carbides, released from the coating, were also observed in the wear fragments. The results of this study can be useful for improving the protective coating and consequently help in reducing particulate matter emission further.

The tribological behavior and the related airborne particles emission of three copper-free automotive friction materials are investigated. The tests were conducted using a pin-on-disc tribometer equipped with a specifically designed clean-enclosure chamber for the emission measurement. Particle number concentration from particle size 0.3 µm up to 10 µm and the mass of emitted particles between 1 µm to 10 µm were measured. Particular emphasis was given to the chemical composition of the bulk materials, the friction layers and the emissions, in order to explain the acting wear mechanisms, and the recorded emission of airborne particles. The results indicate that the recorded emissions do not correlate with the friction coefficient and the wear rates, since the wear mechanisms exert a different influence on the tribological and emission behavior of the materials under study.

Inconel 718 is a popular superalloy with high strength and corrosion-resistance properties that make it widely used in aerospace and nuclear industries. The conventional production of Inconel 718 powder feedstock involves a considerable amount of energy consumption and greenhouse gas emissions. This study investigates the feasibility of using atomized metallic scrap to produce Inconel 718 powder feedstock, as a more sustainable alternative. Life cycle assessments showed that this approach reduces carbon footprint and energy consumption by more than 90% compared to conventional production methods. The study also carried out comparative microstructural, static and fatigue analyses to address concerns about potential impurities in the secondary-source powder feedstock. The results indicate that this secondary-source powder feedstock can achieve properties comparable to those of conventionally processed powders, with negligible loss in ductility and fatigue strength, without requiring ad-hoc tuning of the LPBF process parameters. Overall, the findings suggest that using Inconel 718 powder feedstock produced from secondary sources can be a promising and sustainable alternative to conventional production methods.

The hot deformation behavior of four different steels in the as-cast condition was investigated by means of hot compression tests conducted at temperatures ranging from 1100 °C up to 1200 °C, and at strain rates in between 0.12 and 2.4 s−1. The primary focus of this work was to check the possibility to increase the strain rate during the rough preliminary working of the ingots, i.e., to adopt a rough rolling process in place of the more conventional rough forging. The second aim of the research was to study the influence of the different characteristics of these steels in their as-cast conditions on their hot deformation behavior. It was seen that in all deformation conditions, the stress–strain compression curves show a single peak, indicating the occurrence of dynamic recrystallization (DRX). The hot deformation behavior was studied in both the condition of dynamic recovery (DRV), modeling the stress–strain curves in the initial stage of deformation, and DRX. Data of modeling were satisfactorily employed to estimate the flow stress under different conditions of temperature and strain rate. The experimental values of the activation energy for hot deformation, QHW, were determined and correlated to the chemical composition of the steels; a power law curve was found to describe the relation of QHW and the total amount of substitutional elements of the steels. The critical strain for DRX, εc, was determined as a function of the Zener–Hollomon parameter and correlated to the peak strain, εp. A ratio εc/εp in the range 0.45–0.65 was found, which is in agreement with literature data. All this information is crucial for a correct design of the rough deformation process of the produced ingots.