Development and use of an apparatus for tribological evaluation of ceramic-based brake materials

Automotive braking systems normally employ brake discs made of steel or grey cast iron, which are then paired with composite organic brake pads. These types of materials are suitable for use in braking systems with moderate loads, but car manufacturers are tending to design increasing numbers of prestige and sports-class vehicles that need braking systems with more braking power. As a result, new materials are being introduced into braking systems, for example, carbon-ceramic C/C-SiC composites. However, much higher temperatures are generated at these ceramic surfaces, which imply some new requirements for the contact materials and the testing equipment. In this paper, we present a new tester dedicated for evaluating the tribological performance of ceramic-based composites for brake applications that uses reduced-scale samples with conformal contacts. The size of the samples was determined on the basis of the vehicle's speed (0-300 km/h), the contact pressure (0.1-10MPa), the temperature (20-900 degrees C) and the geometrical proportions of full-scale braking systems. Special care was taken over the testing-head design in order to ensure control and measurement of the high temperatures that are generated at the contact. The simplicity and small size of the samples made it relatively easy to perform the various surface analyses. The theoretical background, the mechanical design and the controls of the testing device are presented. The functionality and reliability of the new device and the testing procedures were verified by using two brake-material combinations: conventional grey cast iron against a metal-matrix composite and a carbon-ceramic composite against a metal-matrix composite. The results confirmed our assumptions about the very high temperatures that are generated at the ceramic contacts and the necessity for well-controlled contact conditions. In addition, these first results suggest there are some beneficial frictional properties with a new material combination using in-house-developed MMC pads and C/C-SiC discs. (c) 2004 Elsevier B.V. All rights reserved.