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As some brake engineers believe that brake squeal can be related to pad hardness, friction coefficient or compressibility while others disagree, a study has been undertaken to develop further insights. Two commercial formulas, one low-copper NAO and the other copper-free NAO, were made into disc pads of varying porosity without an underlayer and they were checked for specific gravity, porosity, hardness (HRS and HRR), natural frequencies, compressibility, friction, wear and squeal. With increasing porosity, the hardness and natural frequencies continue to decrease. The compressibility definitely does not increase, but rather slightly decrease or stays the same. The coefficient of friction decreases for the low-copper along with pad and disc wear reduction, and increases for the copper-free along with pad wear increase with no change in disc wear. No obvious correlation emerges between brake squeal and pad hardness, friction coefficient or compressibility.

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.

In a previous investigation, brake squeal was found to be related to disc wear, but not to pad wear or in-stop average coefficient of friction as tested according to the SAE J2522 performance procedure, using Low-Copper NAOs. To further validate the disc wear - squeal correlation, a variety of formulations of No-Copper, Low-Copper and High-Copper NAO disc pads were made and tested to investigate friction, pad wear, disc wear, brake squeal and wheel dust formation. It is found that disc weight loss measured at the end of the burnish cycle of the SAE J2522 (AK Master) is closely related to dynamometer/vehicle brake squeal and vehicle wheel dust formation, and that there is a critical disc wear rate of approximately 1.0 grams for the current brake system, below which brake squeal and wheel dust are minimal.

Copper-free NAO disc pads of passenger cars were investigated for a combination of prior braking conditions and moisture adsorption influencing in-stop friction and noise during low-speed stops, and in-stop-friction during moderate-speed stops. Prior braking conditions and moisture adsorption strongly influence subsequent in-stop friction behavior and noise at room temperature. The low-speed in-stop friction behavior looks totally different from that of moderate-speed stops. The low-speed in-stop friction increasingly oscillates with increasing moisture adsorption and goes down towards the end of a stop, which is accompanied by increasing low-frequency noise. The moisture content needs to be quantified/specified to obtain repeatable/reproducible brake test results as the moisture is an unintended and uncontrolled ingredient of a friction material. As the disc surface roughness increases due to prior braking conditions, the friction coefficient of low-speed stops is found to decrease.