Chapter 15 - High-Speed Tribology of Polymer Composites

Abstract

Abstract Phenolic-based composites for automobile friction materials contain a variety of inorganic fillers and modifiers in addition to the base resin and fibres (mostly asbestos). The friction behavior at low speeds does not provide a sufficient foundation for the prediction of the friction and wear behavior at high speed, since the temperature rise at high speeds causes chemical changes in some of the friction constituents of the composite system. The purpose of this chapter is to provide a clear interpretation of the frictional performance of phenolic-based composites at high speeds. The friction behavior of commercial brake material at high speeds involves changes in the friction coefficient with sliding distance: an initial decrease, then an increase and finally a constant value with frequent fluctuations, with increasing distance. Finally, the friction tends to have a rather constant value although sporadic fluctuations still remain. The final constant friction behavior is closely related to decomposition of the resin due to the temperature rise at the contacting surface. The critical temperature to reach constant friction depends on the sliding speed and normal load. At high-speed friction of composites simply consisting of a phenolic base resin and carbon, glass, or aramid fibres, the amount of fibres affects the friction behavior. Generally, incorporation of fibres in the resin reduces the friction coefficient. Glass fibre is most effective in decreasing the friction coefficient, and relatively higher amounts of fibres are needed for carbon and aramid fibres to minimize the friction coefficient. The fluctuations in the friction disappear when putting fibres into the resin. This result suggests that reduction of the deformation at high temperatures and products of fibre wear during sliding are responsible for the low friction coefficient of the composites at high speeds.