Browse Publications Technical Papers 2019-01-2131
2019-09-15

The Normal-Load and Sliding-Speed Dependence of the Coefficient of Friction, and Wear Particle Generation Contributing to Friction: High-Copper and Copper-Free Formulations 2019-01-2131

Automotive brakes operate under varying conditions of speed and deceleration. In other words, the friction material is subjected to a wide range of normal loads and sliding speeds. One widely accepted test procedure to evaluate, compare and screen friction materials is the SAE J2522 Brake Effectiveness test, which requires full-size production brakes to be tested on an inertia brake dynamometer. For the current investigation, disc pads of two types of 10 different formulations (5 high-copper and 5 copper-free formulations) were prepared for testing on a front disc brake suitable for a pickup truck of GVW 3,200 kg. Each pad had 2 vertical slots, and one chamfer on the leading edge and also on the trailing edge of the pad. One segment of the test procedure looks at the coefficient of friction (Mu) under different brake line pressures and different sliding speeds to determine its stability or variability. In all cases, the Mu is found to be dependent on the normal load and sliding speed, contrary to the commonly called “Amontons-Coulomb’s Laws of Friction”. According to Wikipedia, Guillaume Amontons (1663 - 1705) observed that the force of friction was directly proportional to the applied load, meaning constant coefficient of friction and that the force of friction was independent of the apparent area of contact, while Charles-Augustin de Coulomb (1736 - 1806) observed that when a piece of metal was slid against wood, the coefficient of friction became very dependent on the normal load and the sliding speed and warned about the limitations of Amontons’ findings. Now one wonders how and when the expression of the so-called “Amontons-Coulomb’s Laws of Friction” surfaced as universal laws. According to analysis of the test data generated in this investigation, the average Mu is found to increase linearly with the increasing total wear rates of the disc and the 2 pads (the inboard and the outboard pads) in the case of copper-containing formulations and also in the case of copper-free formulations: each type of formulation has its own linear relationship. The average Mu is found to consist of 2 parts. One part is dependent on the total wear rates of the disc and the 2 pads while the other part, which is the Mu at zero wear, is independent of the wear rates. Also in this investigation, the disc wear rate is found to be directly proportional to the total wear rate of the 2 pads (the inboard and the outboard). As the pad wear is reported to be best described by a power function of the normal load and the sliding speed, a power function becomes applicable to the disc wear rate as the disc wear rate is proportional to the pad wear rate. So the wear dependent part of the Mu becomes a power function of the normal load and the sliding speed.

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