SiC filament/titanium matrix composites regarded as model composites

Abstract

It is generally accepted that silicon carbide CVD-filaments are more suitable than the related boron filaments for reinforcing titanium alloys at medium temperatures. Silicon carbide has a higher refractoriness and chemical inertia than boron. Moreover, the SiC CVD-filaments retain a much higher fraction of their room temperature mechanical performances up to about 800 to 1000° C and could be produced in the future at a lower price. Despite the fact that SiC seems to be less reactive than boron towards most metals, it nevertheless reacts with titanium and its alloys at rather low temperatures (700 to 800° C). Furthermore, silicon carbide when deposited from a gas phase (e.g. methyl chlorosilane and hydrogen mixtures) often contains small amounts of more reactive species such as elemental silicon or carbon. In the same manner, coatings containing elemental carbon have been applied to SiC CVD-filaments in order to reduce their sensitivity to surface abrasion effects. Therefore, SiC filament-titanium matrix composites must be regarded as non-equilibrium systems when they are heated at medium or high temperatures. The occurrence of chemical interactions between SiC and titanium in the solid state, controlled by diffusion, has already been established either from experiments performed on diffusion couples with a plane interface [1, 2] or on fibrous composite samples [3–12]. However, the nature of the phases and the growth mechanism of the filament-matrix (FM) reaction zone remains a subject of controversy. The aim of the present contribution is to analyse, for various well characterized SiC-based CVD-filaments (stoichiometric SiC, SiC with a pyrocarbon coating, SiC with a SiC + C coating) and titanium matrices (unalloyed titanium or Ti-6Al-4V): (a) the FM interaction zone composition, (b) its kinetics of growth in the 700 to 1100° C temperature range, and (c) the growth mechanism. The filaments which have been used here have been carefully analysed previously on a chemical, microstructural and mechanical point of view in Part 1. In the same manner, the results of the present study on FM chemical interaction will be later correlated with those of different mechanical characterizations [13, 14].