Role of the friction layer in the high-temperature pin-on-disc study of a brake material

The tribological behavior of a commercial brake pad material, wear tested under dry sliding conditions against a cast iron counterface disc with a pin-on-disc apparatus, was investigated. Wear tests were conducted at the following disc temperatures: 25 degrees C, 170 degrees C, 200 degrees C, 250 degrees C, 300 degrees C and 350 degrees C. Above 170 degrees C a transition from mild to severe wear was observed. At 25 degrees C and 170 degrees C, the friction layer on the pin surface consists of primary and compacted secondary plateaus. At 200 degrees C and above, a progressive reduction of the pin surface coverage by the secondary plateaus, that are barely present after the 350 degrees C test, is observed. Wear tracks on the discs derive from wear fragments due to the tribo-oxidation of the disc itself and from the wearing out of the pin material. The observed tribological behavior is very much influenced by the thermal degradation of the phenolic binder of the friction material, confirmed by thermogravimetric analyses, conducted on purpose on the pin material. Raman spectroscopy indicated the presence of carbonaceous products on the high temperature worn out pin surface. Although referring to rather extreme and simplified sliding conditions, the results obtained in this study provide useful indications on the role of the thermal stability of the organic component.