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- Englisch (4)
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- Third body (3)
- Ball milling (2)
- Pin-on-disc test (2)
- Brake pad formulation (1)
- Ceramic disc (1)
- Friction film (1)
- Mössbauer spectroscopy (1)
- PMC-pad (1)
- Pad ingredients (1)
- Raman spectroscopy (1)
A modified pin-on-disc test was applied to determine tribological properties of typical brake pad constituents. Ball-milling of these ingredients together with iron oxide and graphite provided model materials displaying the main features of real third bodies. Solid lubricants like graphite affected the friction and wear behaviour of Fe3O4 powders considerably whereas further addition of hard nanoparticles induced only minor effects. This was corroborated by comparison with modelling results. MoS2 played a dual role. Depending on special conditions, this ingredient either reduced or increased friction. The latter could be explained, after nanoscopic characterization, by oxidation and destruction of the wear-protecting tribofilm.
Different, partly complementary and partly redundant characterization methods were applied to study the transition of magnetite, graphite and MoS2 powders to mechanically alloyed nanostructures. The applied methods were: Transmission electron microscopy (TEM), Mössbauer spectroscopy (MS), Raman spectroscopy (RS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The main objective was to prepare a model material providing the essential features of a typical tribofilm forming during automotive braking, and to assess the impact of different constituents on sliding behaviour and friction level. Irrespective of the initial grain size, the raw materials were transferred to a nanocrystalline structure and mixed on a nanoscopic scale during high energy ball milling. Whereas magnetite remained almost unchanged, graphite and molybdenum disulphide were transformed to a nanocrystalline and highly disordered structure. The observed increase of the coefficient of friction was attributed to a loss of lubricity of the latter ingredient due to this transformation and subsequent oxidation.
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.