Abstract
Large diameter filaments (100–150 μm in diameter) made by chemical vapor deposition (CVD) of two ceramic materials (i.e. boron and SiC) on a heated tungsten or carbon core are compared from a mechanical and chemical standpoint. The most interesting of the filaments studied have received a rather thick surface coating (1–3 μm) which is made of boron carbide for B(W) filaments and a sequence of pyrocarbon and silicon carbide layers for SiC filaments. The mechanical behavior of the filaments in tension is explained on the basis of a Weibull statistics approach as well as a fracture analysis. Failure appears to be mainly controlled by surface defects, a feature which emphasizes the protective role played by the coating. Annealing at high temperatures (i.e. 800–950°C) in the presence of titanium shows that coated filaments have superior behavior. The coating acts in fact as a consumable sacrificial material, the strength of the filament remaining unchanged as long as the coating is not totally consumed by chemical reaction with titanium. Modern CVD filaments appear to be the most suitable ceramic reinforcements from a fundamental point of view.
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