Compatibility between SiC filaments and aluminim in the K2ZrF6 wetting process and its effect on filament strength
Résumé
The effect of the various steps of the K2ZrF6 process, used to enhance the wettability of SiC (or C) fibres by liquid aluminium, on the strength of the reinforcement, has been studied on model materials. Groups of 45 uncoated CVD SiC filaments (100 μm in diameter) were submitted to three treatments representative of the process, viz. (1) coating with a K2ZrF6 layer of variable thickness, (2) annealing of the coated filaments at 650°C in sealed tubes and quenching, and (3) embedding the coated filaments in aluminium by hot pressing. After each step, the filaments were extracted chemically and tensile tested. The UTS data were treated statistically on the basis of a two-parameter Weibull function. Steps 1 and 2 do not significantly modify the filament strength. Step 3 yields a more significant strength loss owing to the formation of Al4C3, related to the chemical reaction between SiC and aluminium. The kinetics of growth of Al4C3 is faster than for untreated filaments mainly as a result of the cleaning effect of fluoride species on the SiC/Al interfaces. Thus, the latter filament strength loss is more an indirect consequence than an intrinsic feature of the process. It could be overcome by utilizing filaments coated with a soft material (e.g, pyrocarbon).