A Comparison of Methods for Characterizing Brake Pad Surfaces

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The brake industry is currently on the search for lighter, corrosion resistant and more eco-friendly brake systems. Apart from health and environmental issues, the main drivers for this development are the megatrends of electrification and autonomous driving. Amongst others, this means that a lot of novel materials are being tested to exchange or improve the currently most used cast iron brake discs. Since the frictional and wear performance is a property of the entire brake system, new brake disc materials often demand an adaptation of the brake pad composition as well. The reason for this lies in the tribological processes taking place during braking. It is known that the real contact area between a brake pad and disc is only a fraction of the size of the apparent pad area and is confined within so called contact plateaus.

This study compares the method of image segmentation that utilizes a light microscope to capture the contact plateaus of a brake pad surface with a different method that uses a focused ion beam for the detection of the contact plateaus.

The output of both methods is a high contrast image of the brake pad surface that shows the contact plateaus in white and the non-contact areas in black. The same area of a brake pad is captured by both methods and processed in a MATLAB script that compares the two images and determines the real contact area. The idea is to use the method of image segmentation using light microscopy as a standard method, because this method will allow for a fast and automated capturing of larger areas of the brake pad surface, allowing for a more global characterization. The validation of this fast method is done via the more complex but also more reliable FIB-method.

EDX (Electron Dispersive X-Ray) maps obtained through Scanning Electron Microscopy are taken from the same area of the brake pad to also receive information on the chemical composition of the contact plateaus. The EDX maps are then overlaid over the black and white images. This combination will help to fully characterize the contact area, allowing conclusions to be drawn on the tribological processes that take place during braking.

Combining this ex-situ evaluation with friction and wear results will allow a more targeted and therefore faster development of new friction materials that address the current and future demands of the automotive industry.

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