Sriwiboon, M., Rhee, S., Kaewlob, K., Tiempan, N. et al., "A Study of Pad Properties vs. Friction, Wear and Brake Squeal: Processing/Porosity Effects," SAE Int. J. Mater. Manf. 10(1):26-33, 2017, doi:10.4271/2016-01-1915.
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. After the pads were compressed under 30, 100 and 160 bars, the hardness of the pad surface is found to increase, suggesting permanent compaction under the pressures while the interior of the pad, 5 mm deep, shows slightly increased hardness in the case of 18% and 22% porosity, not in the case of 13% porosity. The results raise a question about the meaning and usefulness of compressibility measurements.