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The nature of braking friction is extremely complex and a deeper understanding of the physical mechanisms that govern the energy dissipation at the interface of friction pairs is an important tool to create an even deeper knowledge of tribological behavior of friction material. Friction brakes need to transform kinetic energy into heat: a complete knowledge of thermal effects during this process in every brake component is an essential part of brake design. As referred to brake pads, the analysis of dynamometer testing data highlighted experimental evidence related to thermo-mechanical effects, such as the different wear resistance capabilities of material classes (NAO and Low Steel). As is well known in the industry and already published, we observed that tribological characteristics are not constant under all testing conditions and they strongly depend on temperature being the direct consequence of kinetic energy dissipation.

Brake pad material formulation and disc microstructure/composition plays a mutual role during wear test due to the third body layer (TBL) formation and its relative evolution due to temperature change. Nevertheless these ones could influence corrosion behavior. In this study we investigated the effect of rotors characteristics on wear and sticking behavior. Rotor and brake pad microstructure were analysed with optical, metallographic and scanning electron microscope to understand the surface and TBL evolution (using different thermal preconditioning) taking into consideration also the copper role during the different wear stages. During this preliminary study we were able to find out different copper smear morphologies depending on test conditions and rotors features.

An adhesion phenomenon between brake pad and rotor frequently appears as a result of prolonged static exposure to corrosive environments. In these highly oxidative conditions, electrochemical reactions occur on the gray cast iron brake rotor surface to produce iron oxide(s), which can then penetrate the brake pad surface porosity, causing adhesion of the brake pad(s) to the rotor. In some instances the shear load necessary to detach the brake pad from the rotor is sufficiently high and becomes a real issue in the field. The corrosive mechanisms and magnitude of material interactions involved in this issue are very complex. These complexities are in part due to the heterogeneity of the rotor and friction material compositions, but also brake geometry, loading conditions, and environmental variations are large contributing factors.

The friction performance of a Disc Brake Pad is even more required to present stable mu behavior in various environmental conditions such as different temperature and humidity. Interaction between compositional variables (raw materials) and environmental conditions cannot be revealed by a simplistic approach without taking into account their mutual interactions. Thereby is necessary a "crossed" design able to combine mixture components with environmental factors. This paper reports the mu behavior of a commercial Brake Pad Formulation in two different environmental conditions (winter condition, e.g., low humidity, and summer condition, e.g., high humidity) through a Combined Design of Experiment. The design was defined by the variation of three mixture components (Al₂O₃, Graphite and Sulfides) of the Brake Pad Formula according to a Response Surface Method (RSM). The μ behavior has been evaluated on a full-scale dynamometric bench test (AK-Master) with climatic control.

This study investigates two evidences we observe from the field about metal pick up (MPU) generation on passenger car disc brake pads. The first evidence is that Low-steel (LS) materials usually present MPU issues in wet conditions whereas NAO materials mainly give MPU in dry conditions. A deep energy dispersive X- ray fluorescence (XRF) analysis is performed on LS brake pad surface after wet and dry MPU car testing, while a DoE on NAO material is carried out in order to investigate the effects of some compound on dry MPU generation. On the basis of our results, and according to previous literature works, dry and wet MPUs generation mechanisms are proposed. The second evidence is that some rotors are more prone to MPU generation than others. Vehicle and dyno MPU generation tests have been performed on different rotors using the same brake pad material: a first microstructural analysis of the rotors is reported.

Brake pads to be used in the automotive sector are complex mixtures composed of several raw materials of different types (metals, resins, abrasives, lubricants, rubbers…) which undergo mechanical and thermal treatment during their fabrication. Because of the large number of variables affecting the properties of these materials a customization is needed for each specific application, which increases the cost of the final product. In this study a machine learning approach based on regression trees theory has been adopted aiming to support the formulation of friction materials. A model has been constructed to correlate material composition, fabrication and application parameters to the tribological behavior of brake pads tested on dyno benches. The validation of the model has been carried out with reference to a dataset composed of 1000 patterns involving 130 input variables and using as target output the mean friction coefficient measured during standard AK-Master test.

The aim of this work is to characterize the formation of metal pick-ups onto Automotive brake pads, that can lead to major wear problems for the friction material and for the rotors. The characterization of these metal pick-ups has been obtained by means of microscopy techniques (optical microscope and SEM analysis), by X-ray diffraction and thermal analysis, while it has been tried to evaluate the influence of morphological and compositional parameters (like amount of lubricants and of organic compounds) of the pad on the pick-ups formation by performing “true-scale” simulations following specific procedures.