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In order to obtain a better understanding of the role of tribofilms during automotive braking, their structures were investigated and the essential features identified.
Next, different ingredient combinations were studied by preparing some model materials with simpler compositions than real tribofilms. A test method was developed for verifying the tribological properties of the model materials and for comparison with results obtained with numerical sliding simulations of such structures. Prerequisites of good brake Performance properties were identified. Although the coefficient of friction could be varied in a wide ränge, smooth sliding conditions could only be achieved with values smaller than 0.4. Tests with artificial third body powders turned out to be useful as screening method for the selection of raw materials for brake pad formulations.
A way to increase the application range and the lifetime of metallic substrates is their protection by coatings. Here the effect of alumina coatings on common steel substrates having different Chromium content is examined. The background of the investigations is to expand the application possibilities of mechanically good steels to higher temperatures and corrosive atmospheres in the framework of the European project POEMA (“Production of Coatings for New Efficient and Clean Coal Power Plant Materials”) which was introduced to identify materials that can withstand the aggressive conditions arising during the oxyfuel process in modern coal power plants. This process is one possibility to reduce the CO2 emission by firing the coal in oxygen and recirculated flue gas.
Substrate materials were P92 (9% Cr), K44X (19% Cr), and HR3C (25% Cr). The alumina coatings were deposited by a sol-gel-process using boehmite, this is relatively simple and offers application possibilities for a wide technical range without special surface preparations. All samples were dried for 30 minutes at 650° C to remove the organic components and to start the crystallization process of the alumina. For P92 and HR3C the following conditions simulated the oxyfuel process: 650°C in wet flue gas for 300h for the steel P92, the same conditions for 2000 hours for the steel HR3C. K44X was tested at 900°C in laboratory air for 500 hours. As well after drying as after testing TEM characterizations were performed, where the interfaces between steel and coating are of special interest to characterize the microstructure, detect failure mechanisms, and identify diffusion and crystallization processes and so to understand the mode of action of the alumina coating.
The TEM samples were prepared by the lift-out-technique using a FIB Quanta 3D and were investigated in a STEM JEM2200FS at 200kV.
After the coating and drying process all steels show chromium oxide layers directly at the steel surface, they vary in thickness and are island shaped in the case of the steel K44X. Chromium depletion arose more or less in the superficial zones of all samples. After the long term tests K44X showed breakaway oxidation while P92 and HR3C presented intact interfaces. This reveals: Beside the positive impact of the coating not only the Chromium content of a substrate is essential for the formation and self-healing of protective chromium oxide layers during the service time of steels. One has to consider the possibility of the steel to allow a continuously Cr diffusion to the interface and so e renewal of the Cr-oxide layer.
A model system was used to simulate the properties of tribofilms which form during automotive braking. The model system was prepared by ball milling of a blend of 70 vol.% iron oxides, 15 vol.% molybdenum disulfide and 15 vol.% graphite. The resulting mixture was characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and various transmission electron microscopic (TEM) methods, including energy dispersive X-ray spectroscopy (EDXS), high resolution investigations (HRTEM) with corresponding simulation of the HRTEM images, diffraction methods such as scanning nano-beam electron diffraction (SNBED) and selected area electron diffraction (SAED). It could be shown that the ball milling caused a reduction of the grain size of the initial components to the nanometer range. Sometimes even amorphization or partial break-down of the crystal structure was observed for MoS2 and graphite. Moreover, chemical reactions lead to a formation of surface coverings of the nanoparticles by amorphous material, molybdenum oxides, and iron sulfates as derived from XPS.
In November 2007, OECD’s Working Party on Manufactured Nanomaterials (WPMN) launched the Sponsorship Programme for the Testing of Manufactured Nanomaterials (hereafter the Testing Programme). The objective was to conduct specific tests, relevant to human health and environmental safety endpoints, on a variety of manufactured nanomaterials (MN). The outcomes of the Testing Programme were intended to assess the applicability of the existing test guidelines1 to nanomaterials, as well as to provide useful information on any intrinsic properties of MNs, which are different from the same bulk material with greater external dimensions. Understanding the properties of NMs is crucial to choose appropriate strategies for hazard identification, risk assessment or risk management measures. The Testing Programme involved delegations from OECD member countries, some non-member economies and other stakeholders. The broad international representation, from a range of delegations enabled the programme to pool expertise and resources without which this programme would not have been possible.