Chemical and physical properties of carbon as related to brake performance

Abstract

The two objectives of this paper are: (a) to review the literature relevant to the structure, properties and applications of carbon-carbon (C/C) disc brakes, and (b) to present aspects of carbon science relevant to an understanding of the operational behaviour of such brakes. The literature describes numerous studies relating structure and composition of (C/C) disc brakes to wear mechanisms and performance. C/C brakes are manufactured from carbon fibres and a matrix carbon, usually an isotropic carbon from a resin, an anisotropic carbon from coal-tar pitch, or an isotropic carbon from pyrolysis of methane (CVD or CVI). Coefficients of friction depend upon the weave of the fibres, the matrix carbon, heat treatment temperatures, ambient gases such as air, water vapour and carbon dioxide, the testing conditions used (low- or high-speed), and the generation of wear dust. Wear generates debris (wear dust) from the surface which is then further comminuted during braking. Wear rates are also associated with gasification of both the brakevsurface and of wear dust. Mechanisms of generation of wear dust are summarised, and dull and lustrous wear dust surfaces are reported. Structures in the various forms of carbons are described and the special characteristics of C/C disc brakes are set out. The physical properties of graphite of dominant importance to brake performance are cleavage of the graphite crystal, and its thermal conductance and thermal capacity. Operational changes to braking performance are modelled. Carbon gasification reactions are explained, as well as the role of surface oxygen complexes and their influence on the absorption of water vapour which then, as a film, acts as a lubricant. The roles of wear dust during braking are elaborated upon.