Thin coatings on carbon fibers as diffusion barriers and wetting agents in Al composites

Abstract

This paper describes the preparation of CVD coatings (Ta, TiC, TiN, TiNC and SiC) on carbon fibers and the effect of the deposition parameters on the mechanical properties of the fibers. Ta does not influence the fiber strength, whereas for the other coating materials the deposition conditions must be optimized in order to retain the original fiber properties. In view of the application of such coated carbon fibers in composites, their wetting behavior with liquid aluminum as well as the behavior of the coatings as diffusion barriers were studied. The coatings are instantly wetted only at temperatures so high that a partial penetration of aluminum through the thin coatings and hence a reaction at the surface of the carbon fiber leading to the formation of aluminum carbide become inevitable. The fiber strength of high performance fibers is then considerably impaired although low strength fibers are hardly affected. Wetting by liquid aluminum below 800°C can be achieved by using additional thin nickel layers. In this way the infiltration of coated carbon fiber bundles can be realized and theoretical fiber strength yields in accordance with the rule of mixtures can be obtained. This improved wetting behavior was found not to depend on the type of coating. The barrier effect of the coatings allows heat treatments at up to 600°C for short times with high fiber strength yields. Heat treatments of the order of 100 h, however, gradually decrease the composite strength. In order to retain high fiber strength during long heat treatments at high temperatures, a minimum coating thickness of some microns is needed. The preparation of such thicker coatings is discussed in detail. With such a procedure, one can obtain reinforcing elements comparable with commercially available boron or silicon carbide monofilaments.