Establishment of Scientific Correlation Between Automotive Field Testing and Scaled-Down Bench-Top Friction Brake Testing Strategies

Abstract

Increasing operational costs related to testing of brakes and the friction materials has been a rising concern for the industries. Particularly the most recent impact related to regenerative braking and increased use of electric, hybrid and autonomous vehicles as well as the related changing role of the friction brake open further opportunities for development of more advanced and sustainable brake materials and brake designs. Although this is not aimed to replace the currently accepted �large scale testing strategies,� previous work demonstrated that the scaled-down brake dynamometer testing approaches and utilizing �bench-top� testing can yield reasonable information facilitating research and development [1-3]. The goal of this paper was to explore whether reasonable correlation exists between the FMVSS 135 standardized field test and a �scaled-down� bench-top test, designed by using scaling laws. Selected sections of the Federal Motor Vehicle Safety Standard 135 (FMVSS 135) were adopted for both the field test and the scaled-down bench-top test for vehicles with gross vehicle weight rating (GVWR) of 3500 kg or less. This presentation concentrates on data obtained for the Ford F150 2009 truck. The commercially available semi-metallic (SM) aftermarket brake pads were rubbed against commercially available Ford F150 aftermarket brake grey cast iron rotors coated with patented ceramic coating technology [4]. The study was performed by using front wheel brakes and the wear mechanisms, morphology and chemistry of friction surfaces of both field tested and bench-top tested samples were analyzed using scanning electron microscopy in the secondary and back-scattered electron modes (SEM, Quanta FEG 450 by FEI) with the capacity to perform qualitative surface topography inspection. The microscope was also equipped with the energy dispersive x-ray microanalysis (EDX, Inca System). Although the absolute values of the coefficient of friction levels were not identical, similar trends were observed in the friction data obtained from the field tests and the bench-top tests, respectively. The quantitative differences were ascribed to the inherent differences in temperature development during the friction. While the real brake is heated as a consequence of applied friction engagements, the bench-top tester �has to use� a heating chamber to achieve similar temperatures. Surface analysis results show different morphology and chemistry of the friction surfaces inspected after the field tested and the bench-top tested pads and rotors were compared. The detected findings/differences in chemistry and morphology correspond well to the detected differences in absolute values of the friction levels and to the detected differences in wear and wear mechanisms. The dominant role of the friction layers generated during both field and bench-top tests was further confirmed and it became apparent that the character of the friction layer can be correlated to brake performances during different tests. This will further allow for modification of bench-top testing strategy and a combination of scaling laws approach. Furthermore, the information obtained from the friction surface analysis will allow for further improvement of bench-top testing reliability, similar to previously published knowledge [5].