Brake materials represent an important source of air pollution, especially in urban areas, where they can contribute to approx. 21 % of the traffic-related particulate matter emission. For this reason, the design of new brake materials with low emissions is a topical issue. In addition to low emissions, the design of new friction materials has to ensure excellent performance with stable coefficients of friction and low wear rate. Due to the several requirements that these materials need to fulfill, their development and testing are complex and intercorrelated. Good performance and low emission strongly depend on the mechanisms acting at the disc-pad interfaces. In this thesis, a brake dynamometer testing protocol is developed to better understand the relationships of the braking parameters with the brake performance and emission behavior, correlating them with the surface characteristics. The surface characteristics were investigated with a-posteriori analysis, in terms of extension of the contact area, degree of compaction of the wear particles and relevant composition. The work is focused on the bedding process and the influence of the braking parameters on the frictional, wear and emission behaviors. Regarding the bedding process, run-in, transition stage and steady states were identified as concerns the frictional, wear and emission behaviors. The frictional behavior gets stabilized by the extension of the secondary plateaus, whereas the wear and emission behaviors are stabilized as their degree of compaction increases. The influence of pressure and velocity under mild sliding conditions were studied for a low-met and NAO material, the two most common types of friction materials. The low-met material featured a more stable and higher friction coefficient and lower wear and emissions than the NAO material. The wear behavior is strongly affected by pressure for the NAO material, and for the low-met material, velocity is very influential. Emissions follow a cube relationship with velocity for both materials. The significant differences in the observed behaviors are explained in terms of the different features of the surfaces. The NAO material featured a smooth and uniform surface, with higher coverage than the low-met material, on which steel fibers play important adhesive and abrasive actions. From tests under mild sliding conditions of several friction materials sliding against cast-iron discs, a linear relationship is found between the specific wear rate and the emission factor. This relationship identifies a wear rate below 2.5 10-14 m2/N complying with the Euro 7 limitation of 3 mg/km/vehicle after 2034. Among the friction materials sliding against cast iron discs, the NAO material and only one friction material displayed an emission factor below the limit of 3 mg/km/vehicle. In addition, the emission factor of low-met material sliding against a cermet-coated disc was lower than this limit. These observations confirm that the NAO materials and coated discs are effective systems to mitigate emissions, whereas further efforts are required to improve the emission behavior of low-met materials. Interestingly, the low-met materials with a reduced presence of secondary plateaus featured higher wear and emissions. Regarding the brake performance, under severe sliding conditions, the NAO material displayed worse frictional and wear behaviors than the reference low-met material. For high-pressure ranges, the effect of pressure is to cause a monotonic decrease in the friction coefficient. The effect of temperature on the friction coefficient causes an increase in the friction coefficient when the tribo-oxidative processes are contained up to 300 °C. For combinations of high velocity and temperature, the tribo-oxidative processes are high enough to form a thick glaze layer on the surfaces. The glaze layers were correlated to a lubricating effect, or fade effect, at disc temperatures above 400 °C, especially when their extension covered the steel fibers. The cermet-coated disc displayed the same fade behavior at high velocity-temperature values, although at low velocities and high temperatures, friction instability was observed and related to larger but fewer patches originating to a significant extent from material transfer from the disc. The friction instability in the coated disc was ascribed to the different tribo-oxidative behavior in the formation of ‘glazes’ due to the low source of iron in the disc material.

A metal injected component was produced by overmolding technique. To reach a good result differebnt powder matching were studied. The final mechanical properties and corrosion resistance of the interface microstructure were investigated.

Binderless tungsten carbide (BTC) ceramics are inherently difficult to process and very brittle. Most consolidation techniques for processing pure WC powder require long sintering times and intense energy consumption. High-T pressureless and pressure-assisted sintering processes often lead to low-quality and coarsened microstructures, thus limiting the use of WC ceramics to few niche applications. Field-assisted sintering techniques (FAST), like spark plasma sintering (SPS), significantly improve the densification of fine and ultrafine WC powders. However, SPS requires high current outputs and expensive apparatus. SPS ceramics still lack adequate toughness to extend the use of BTC components in heavy-duty applications requiring reliable load-bearing capability and/or resistance against rapid and unexpected impacts or temperature drops. This research work explored a new consolidation route capable of boosting the mass transport phenomena (accelerated sintering) and, simultaneously, introducing new microstructural features. The process called flash sintering (FS) offers great potential in accelerating diffusion phenomena and altering the crystallographic and/or the defect chemistry of the sintered ceramics. Many scientific studies reported structural alterations, enhanced plastic flow and material softening by introducing “out-of-equilibrium” characteristics. Currently, FS technology requires, for its activation, a negative dependence of the electrical resistivity with temperature (NTC) of the material to be sintered. This is a universal requirement for the flash event to occur thus theoretically inhibiting the flash sintering of conductive materials with a positive temperature coefficient for resistivity (PTC), like metals or WC. In the present work, we reported how during electrical resistance sintering (ERS) experiments conducted on pure WC nanopowders, a flash event was triggered during the first seconds of the process. This was demonstrated to occur thanks to the different evolution of the electrical properties of a granular compact with temperature. WC powders possess an initial NTC behaviour which can activate a transitory thermal runaway phenomenon which makes the activation of a flash event in these materials possible, intense enough to allow ultrafast densification in less than 10 s. This breakthrough allows to verify whether and how the flash event modifies the final sintered material. FS and SPS sintered ceramics were compared in their microstructural, physical and mechanical properties, thus pointing out how some peculiar modifications are exclusively present in the flash-sintered material. FS can stabilize the WC1-x metastable phase after cooling to room temperature, and this was demonstrated to alter the high-temperature deformation of WC micropillars during compression. In addition, FS BTC are inherently softer with respect to SPS ones, resulting in higher fracture toughness and slightly lower hardness. Even if not final, the results indicate how the flash sintering of WC can be explored further to process engineered BTC ceramics with an optimized hardness/toughness ratio and an enhanced deformability.

Peak profiles in X-ray Diffraction (XRD) patterns from nanocrystalline materials are affected by static and dynamic disorder which is specific of the size and shape of the nanocrystalline domains. Owing to their intrinsic differences, the two types of disorder can be separated, providing independent information from the modelling of the XRD patterns. In the present thesis a model for the static strain created by the nanoparticle surface is proposed. The model is built within the frame of the Whole Powder Pattern Modelling (WPPM) approach for XRD line profile analysis, developed at the University of Trento in the past 20 years. The WPPM approach is decribed in details. Based on a complex Fourier Transform of the diffraction profiles, the model leads to general equations to be used with the WPPM approach to represent the distorted atomic configuration with respect to the reference bulk one. The model was also implemented in TOPAS, a commercial and very popular software, developing a specific macro allowing a larger community of users to benefit of this new opportunity of studying nanocrystalline materials. The thesis work also extended to a more traditional and general description of strain broadening of XRD peak profiles, involving invariant forms under the Laue group symmetry operations of the material under study. As for the dynamic strain, the fundamentals of the Thermal Diffuse Scattering (TDS) contribution to the peak profiles are reviewed. Starting from the original work of B.E. Warren, the theory is generalized to account for surface effects, leading to a particular model developed recently at the University of Trento. This model was thoroughly reviewed and corrected. To test the model a parallel computer code in C was written, exploiting Molecular Dynamics simulations for obtaining reliable and independent estimates of static and dynamic disorder in nanocrystals.