Disc Pad Physical Properties vs. Porosity: The Question of Compressibility as an Intrinsic Physical Property

Disc pad physical properties are believed to be important in controlling brake friction, wear and squeal. Thus these properties are carefully measured during and after manufacturing for quality assurance. For a given formulation, disc pad porosity is reported to affect friction, wear and squeal. This investigation was undertaken to find out how porosity changes affect pad natural frequencies, dynamic modulus, hardness and compressibility for a low-copper formulation and a copper-free formulation, both without underlayer, without scorching and without noise shims. Pad natural frequencies, modulus and hardness all continuously decrease with increasing porosity. When pad compressibility is measured by compressing several times as recommended and practiced, the pad surface hardness is found to increase while pad natural frequencies and modulus remain essentially unchanged. However, there is no consistent pattern in compressibility change with increasing porosity, and thus a question arises on the validity of compressibility measurement as an intrinsic physical property measurement. Also after 12-months of ageing at room temperature, all the properties are found to change significantly, but property change trends with increasing porosity remain the same except for compressibility. A large number of samples were prepared and measured. The results are presented and discussed.