CHEMICAL AND PHYSICAL-PROPERTIES OF CARBON AS RELATED TO BRAKE PERFORMANCE

The two objectives of this paper are: (a) to review the literature rel evant to the structure, properties and applications of carbon-carbon ( C/C) disc brakes, and (b) to present aspects of carbon science relevan t to an understanding of the operational behaviour of such brakes. The literature describes numerous studies relating structure and composit ion of (C/C) disc brakes to wear mechanisms and performance. C/C brake s are manufactured from carbon fibres and a matrix carbon, usually an isotropic carbon from a resin, an anisotropic carbon from coaltar pitc h, or an isotropic carbon from pyrolysis of methane (CVD or CVI). Coef ficients of friction depend upon the weave of the fibres, the matrix c arbon, heat treatment temperatures, ambient gases such as air, water v apour and carbon dioxide, the testing conditions used (low- or high-sp eed), and the generation of wear dust. Wear generates debris (wear dus t) from the surface which is then further comminuted during braking. W ear rates are also associated with gasification of both the brake surf ace and of wear dust. Mechanisms of generation of wear dust are summar ised, and dull and lustrous wear dust surfaces are reported. Structure s in the various forms of carbons are described and the special charac teristics of C/C disc brakes are set out. The physical properties of g raphite of dominant importance to brake performance are cleavage of th e graphite crystal, and its thermal conductance and thermal capacity. Operational changes to braking performance are modelled. Carbon gasifi cation reactions are explained, as well as the role of surface oxygen complexes and their influence on the adsorption of water vapour which then, as a film, acts as a lubricant. The roles of wear dust during br aking are elaborated upon. (C) 1997 Elsevier Science S.A.