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Copper-free disc pads of 9 different compositions were made using a traditional hot molding process and tested to study frictional behavior. It is found that the friction coefficient consists primarily of two parts; one part controlled by the plastic deformation of the friction surface region of the disc and pad, and the second part controlled by the total wear of the disc and pads. As the plastic deformation and the wear are non-linear with respect to the load and sliding speed, the friction coefficient becomes a non-linear function of the load and speed. Under moderate braking conditions, the plastic deformation part is more significant in determining the friction coefficient while under more severe braking conditions, the wear contribution becomes more significant. The frictional behavior of a fade cycle is explained, and the correlation between brake squeal and disc wear is confirmed.

One low-copper formulation and one copper-free formulation were made into disc pads, and both of them were cured under 4 different conditions. These pads had no backing layer and no scorched layer. Pad thickness, dynamic modulus and natural frequencies were continuously monitored over a period of 12 months. After 12 months at room temperature, pad thickness, dynamic modulus and natural frequencies all increased to higher values. The low-copper formulation increased relatively rapidly during the first 60 days and the copper-free formulation increased relatively rapidly for the first 90 days, and then slowly thereafter. Two competing processes are found to be taking place simultaneously; internal stress relief leading to pad expansion and cross-linking of the resin leading to pad shrinkage. As the pad properties are changing continuously, the timing of property measurement becomes an important issue for quality assurance.

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.