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- Automotive braking (3)
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Organisationseinheit der BAM
TEM study on the effect of machining on the microstructure of a soft magnetic Ni76Fe17Cu5CrMo2 alloy
(1991)
The role of grain size and ferrite substructure in respect to mechanical properties of an HSLA steel
(1986)
Several heat treatments were applied to an HSLA steel on type StE460 (German standard) to produce four ferrite substructures of different strength and at least two different grain sizes respectively. Whereas the ferrite substructure has a strong influence on yield strength the effect of grain size was negligible though probably to some extent masked by different pearlite volume fractions. The different strength levels could be explained by regarding the arrangement of dislocations and vanadium carbonitride particles and their mutual interaction. Increasing both grain size and pearlite volume fraction leads to a remarkable shift of transition temperature T sub 27J which was further enhanced by increasing ferrite strength. 16 ref.--AA
Optical and electron metallography have been used to investigate the development of different microstructures in Armco Fe after rolling reductions between 0 and 90%. Simultaneously, the orientation distribution functions have been calculated from pole figures obtained by X-ray diffraction. Different stages of texture development could be assigned to typical microstructures. Strain inhomogeneities, such as microbands and shear bands, caused some broadening of the texture and a slight deviation of one skeleton line from the predicted fibre texture component. The decomposition of orientations near {111} < 121 > , and the strengthening of {211} < 011 > after high rolling reductions, was not due to mechanical twinning but could rather be explained by assuming {211} < 111 > slip. 30 ref.--AA
Third bodies, also termed friction layers, tribofilms or secondary contact patches, are layers of more or less compacted wear debris between pads and rotor of a disc brake. Our approach of assessing the sliding behavior and friction properties induced by third bodies has been: i) structural characterization after AK-master test procedure, ii) sliding simulation of model structures similar to the observed ones but with simpler and well defined compositions, and iii) verification of simulation results by pin-on-disc tests with artificial third bodies showing the same microstructures and compositions as the model structures. The idea was to simulate structure formation during real braking conditions by high energy ball milling of appropriate powder blends. The final outcome of numerous parameter studies was that a third body containing 15 vol% soft ingredients and 0-20 vol % hard ingredients, both distributed homogeneously in a nanocrystalline iron oxide matrix, should be most desirable for braking. This general description of the optimum third body structure and several parameter studies performed by modelling enabled us to interpret a number of features of the AK-master test procedure, such as i) friction evolution during bedding, ii) the role of solid lubricants in respect to the initiation of smooth sliding behavior, and iii) friction evolution during a single braking event (in-stop behavior).
In the paper the method of discrete modeling (movable cellular automata method) and combined discrete-continuous description of the simulated medium are used to analyze processes occurring in the local contact of the automotive brake system. The characteristic size of the considered region is 1.5 ?m. The following contact situation is simulated: steel fiber coated by an iron oxide film as the brake pad and pearlitic steel also coated by an iron oxide layer as the disc. On the assumption of oxide layer wearing we simulate the iron oxide - iron oxide, iron oxide - metal and metal - metal contacts.
The calculation results for the friction coefficient for various contact situations give quite adequate values. For example, for the oxide - oxide system the calculated coefficient is approximately equal to 0.4, while for the metal - metal contact the obtained value varies from 0.7 to 0.9. Analysis of a set of the obtained results allows concluding that oxide is formed more rapidly than the sliding layer, which in turn makes the friction coefficient value stabilized.