Automotive Brake Materials: Characterization of Wear Products and Relevant Mechanisms at High Temperature

Wear is an ubiquitous phenomenon affecting an extremely wide number of technological system, often determining their premature failure. In this regard, wear and friction behavior of friction materials and the characterization of wear debris from brake disc system is an important step to understand the dominant wear mechanisms active in a given tribological system, in order to improve its performances and to increase the expected lifetime. In the thesis, four tribological task has been performed, under the code name Case I, II, III & IV. This thesis present the work on the development of a characterization methodology of a wear debris from brake pad-disc system, M1 and M2 friction materials at elevated temperatures and study of the wear and frictional behavior of a heat treated cast iron disc. In Case I, the dry sliding behavior of two friction materials (M1 & M2) have been investigated. The sliding tests were carried out on a pin-on-disc test rig, using a cast iron disc as a counterface, under mild conditions (the applied nominal pressure was 2 MPa and the sliding speed was 3.14 m/s). The results shows that friction material M2 is characterized by a lower friction coefficient than friction material M1, and the friction coefficient is stable during the test. In addition, friction material M2 shows a lower wear rate than M1. The results were explained by considering the characteristics of the friction layer that is established during the test. On the bases of the experimental observations, the lower friction and wear of friction material M2 was attributed to the formation a quite uniform and well compacted friction layer, due to the presence of ingredients, such as Zr oxides, able to form small particles during sliding that are compacted and held together by the presence of metallic ingredients, such as copper. The absence of Zr-oxides in the formulation of M1friction material and the presence, in their place, of hard and abrasives Mg, Zn and Al-oxides, impeded the formation of wide covering friction layer, increasing friction and wear. The different frictional properties of the brake pads determine their driving performances, and the different wear behavior determine their in-service deterioration and also their attitude to emit particulate matter in the environment, which is nowadays a concern of increasing importance. Under the Case II, a streamline characterization protocol for wear debris emitted under wear testing conditions (Case I - M1 friction material) used for disc brake assemblies is presented. An important aspect of the experimental test methodology concerns the powder collection methodology on different substrates: aluminum foil, for a gravitational integral collection, and polycarbonate filters of an ELPI+ impactor equipment, on which particles are selectively trapped, according to their average size. The protocol is based on the application of different materials characterization tools, like scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). The deliberate aim of the study was to identify suitable selection parameters, like specimen availability and average particle size, for an effective and smart application of the mentioned experimental techniques so to optimize testing times and obtain statistical reliable results. The proposed characterization approach could be profitably employed also in other contexts, like environmental and health monitoring, as far as particulate matter, even from other sources than brake systems, is concerned. We extended the work in Case I by investigating further wear mechanisms of M1 & M2 pins at elevated temperatures i.e., 170°C, 200°C, 250°C, 300°C and 350°C, under Case III. The results showed a clear evolution of frictional parameters with temperature. For M1, the working temperature were 155°C, 200°C, 250°C and 300°C, the absence of frictional parameters with temperature and wear behavior of M1 is higher than M2 with one degree higher order of magnitude. Wear tracks on the discs form from the piling up of wear fragments produced both by the tribo-oxidation of the disc itself and from the wearing out of the pin materials. This accumulation of wear debris on the disc surface nearly compensate for the weight loss associated with disc wear. The observed tribological behaviour is very much influenced by the thermal degradation of the phenolic binder of the friction material. The thermal decomposition kinetics was confirmed by thermogravimetric analyses, conducted on purpose on the pin material, and by Raman spectroscopy results, that confirmed the presence of carbonaceous products on the worn out pin surface. For M2, the working temperature were 170°C, 200°C, 250°C, 300°C and 350°C, above 170°C a transition from mild to severe wear was observed. Correspondingly, the friction layers, in particular, the secondary plateaus, which develop on the pins and disc surface during sliding displayed quite different features, as proved by electron microscopy observations and X-ray spectroscopy analyses. As concerns the pins, at 25°C and 170°C, the friction layer consists of primary and well compacted secondary plateaus. At 200°C and above, a progressive reduction of the pin surface coverage by the secondary component of the friction layer and a corresponding thinning of this component are observed. Secondary plateaus are barely present on the samples tested at 350°C. Although referring to rather extreme conditions and simplified sliding conditions, the results obtained in this study provide useful indications on the role that the thermal stability of the organic component may have in determining wear rate in brake systems in which the temperature rise may be induced by actual operational conditions. The Case IV work aims at illustrating the role of conventional heat-treatments on the friction and wear behavior of the above system. Wear rates of both disc and M2 friction material were reduced by almost one order of magnitude when the disc is preliminarily heat-treated and then grinded to remove the surface decarburized layer that forms during the adopted treatment cycle. Heat-treatment and heat-treatment plus ground results in the reduction of the friction coefficient, which was comparatively low for the grounded samples (grinded to remove the surface decarburized layer). The friction and wear behavior along with the contact temperature evolutions were rationalized according to the materials characteristics and the observed wear mechanisms