Some interfacial problems in metal matrix?carbon fibre composites

Abstract

Metals reinforced with continuous graphite (Type I) fibres are prone to compatibility and oxidation problems when they are subjected to high temperature treatments. To prevent such interactions, the feasibility of using evaporated barrier layers has been examined in a nickel matrix. A number of techniques, e.g., tensile testing, thermal-balance determinations and microscopy have been employed to test the usefulness of this approach. Results have shown that 1 µm thick metal coatings either react vigorously with the fibre or allow diffusion of oxygen or carbon to take place sufficiently quickly so as to delay fibre degradation by only a short time interval. Whilst chemically stable nonmetallic coatings tend to prevent diffusion of both carbon and oxygen, they are extremely susceptible to brittle cracking due to differential thermal expansion effects. In this respect, zirconium carbide and boron nitride seem to be affected to a lesser degree than other carbides, nitrides and oxides, and can provide protection over a limited temperature range.