Friction performance of ceramics and carbon fibre reinforced silicon carbide composites

Carbon fibre reinforced silicon carbide composites (Cf/C-SiC) have been increasingly used as rotor materials for light weight automotive brake discs. Despite of the commercial success, the performance related issues such as poor bedding and degraded friction performance under wet conditions have been recently addressed for the Cf/C-SiC composite brake discs. The present work is focused on the bedding and the subsequent wet friction behaviour of the commercial Cf/C-SiC composites, monolithic SiC and Si3N4 ceramics under simulated automotive braking conditions.

The aim of this research is to understand the relation between the friction surface development on the ceramic discs and the friction behaviour, and to provide fundamental guidelines for further development of the surface coating on Cf/C-SiC composites to achieve desired friction performance.

The experimental work included the dynamometer testing of different ceramic discs (SiC and Si3N4)-pads (steel, copper and organic pads) friction couples under both air and wet conditions. The microstructure of the friction surface of the ceramic discs was characterized at different stage of testing and the friction mechanisms were discussed based on the adhesion and ploughing theories. Among these couples, the Si3N4-steel couple exhibited the most sustainable dry and wet friction performance as well as reasonable wear rate. This is largely related to the formation of friction transfer layer (FTL) composed of a mixture of fine metal/metal oxide crystals on the discs surface. For the disc surface without the formation of sustainable FTL, consistently poor wet friction performance was obtained unless it was significantly roughed. TEM results revealed the oxidization of the SiC or Si3N4 ceramic substrates is responsible for the bonding between the disc surface and the deposited metal/metal oxides, while the excessive cracking of SiC when against steel pad is likely originated from pile up of partial dislocations or twining on the basal planes.

The friction behaviour of the commercial Cf/C-SiC composites was also investigated. The results indicated the coefficient of friction (COF) of the Cf/C-SiC composite discs against the organic pads is largely defined by the unique friction characteristic of the SiC regions, where the COF decreases with increasing contact pressure. Under the same braking load, increasing the SiC content on the surface of Cf/C-SiC composites results in lower effective pressure on SiC and hence leads to a higher COF. Under wet conditions, a relative high COF was obtained for the Cf/C-SiC composites at initial stage of braking, when the friction transfer layer was attached on the surface.

With further braking, the friction transfer layer was stripped off, and the COF dropped to below 0.1 due to the formation of the water film. It further validated that the wet friction performance of the ceramics discs can be improved by bonding of the friction transfer layer (FTL) on the top of the disc surfaces. A surface coating composed of SiC and Si3N4 based ceramics has been developed and the coated disc showed significantly improved bedding and wet friction performance over the uncoated ones under lab-scale testing conditions. The improved performance is due to the fast formation of FTL that was chemically bonded on the disc surfaces.