Brake discs on the basis of carbon fibre reinforced SiC-ceramics are appreciated as a light weight high performance alternative to conventional cast iron brake discs. Although efforts are going on to develop ceramic pads of a similar material but with additions of friction modifiers, comfort requirements cannot be reached with a full ceramic system and therefore commercial solutions still are based on polymer matrix composites (PMCs) and in some rare cases on sinter metallic (SM) friction materials.
In previous studies with conventional automotive brakes an iron oxide based friction film on the surfaces of pads and discs was observed. This seemed to be plausible since iron is the major element of the cast iron disc and furthermore is contained in most pad formulations in the form of steel fibres. Therefore it was of great scientific interest to determine the relevance and composition of friction films on ceramic brake discs which do not contain any iron.
Three brake systems (2 with PMC and 1 with SM pads) were investigated in this study. For proprietary reasons the companies did not communicate their formulation recipes, but they agreed to publish the results of microstructural and microanalytical investigations in a generalized form. Anyway, all three pad formulations contained iron either in the form of steel fibres or as powder particles. Friction films were characterized by Light optical Microscopy (LM) and surface analytical methods (SXFA, SEM/EDS) and by cross-sectional methods (FIB, TEM, STEM/EDS, EFTEM).
Surprisingly, the basic structure of friction films was similar for all three systems and again the main constituent was iron oxide of type magnetite mixed with other pad constituents. This result clearly shows that tribooxidation of the ferrous pad constituents has occurred, and that the wear particles are able to form a film which is screening both first bodies. Significant differences between the three systems were observed in respect to film thickness and coverage of the disc surface with the third body film. Whereas the disc of the SM-system was covered almost completely with a ferrous film, only fragments of a film, or deposition of wear debris in a network of cracks were observed for the two PMC-systems, respectively. It seems that the surface structure of the C-SiC disc, i.e. carbon fibre content and number and arrangement of cracks plays an important role in respect to the formation of stable third body layers. Furthermore, incorporation of additional soft and hard constituents from the pad formulation is necessary to stabilize the friction properties of the iron oxide film. Compared to conventional brakes the ratio of additives to iron oxide in the third body is higher for the ceramic discs.
Österle, Werner*, Deutsch, Cornelius, Rooch, Heidi, Dörfel, Ilona - BAM Federal Institute for Materials Research and Testing